Program 29 - "Periodicity and Bonding"

MusicI just don't know how to classify all these elements.I have so many elements, I just can't figure out what to do with them.I put them this way, and it doesn't work.I put them this way, and it doesn't work.These two don't match.I just don't know what to do.There must be some way to classify them.Classify them!I put them like this and they don't work.Hmm nyet!I put them this way.....MusicSilico: "We are back with Science 122, the Nature of Physical Science.This is the only telecourse that shows you how to form strongbonds with your friendly atoms while you classify and categorize them.This is Program 29, "Periodicity and Bonding."Before we're done with this program we will havesummarized the advances in chemistry in the 19th centuryculminating in the Periodic Table of the Elements.

We will have seen how the structure of the periodic tablereflects the quantum nature of electron patterns surrounding atoms.We will have seen why chemical bonds form, and how this allowsfor the formation of several types of chemical bonds,how it explains the unusual properties of water,and how it explains the properties of electrolytes, acids, bases and salts.Here are the objectives for today's lesson.These objectives are also in the Study Guide at the beginning of the lesson.Before you begin to study the lesson, take a few minutesto read the objectives and the study questions for this lesson.Look for key words and ideas as you read.Use the Study Guide and follow it as you watch the program.Be sure to read these objectivesin the Study Guideand refer to them as you study the lesson.Focussing on the learning objectives will help youstudy and they'll help you to understand the important concepts.Compare the objectives with the study questions for the lessonto be sure that you have the concepts under control.

You know, two of mankind's greatest intellectualachievements are the classification of the elementsand the theory of the chemical bond.We're going to be considering both of those in this program.It seems like a rather large charge, but with something thatwe need to do, because it really ties everything together.These two things, the periodic table and the theoryof chemical bonds, represent our modern understandingof the chemical nature of matter and they allow us to manipulateit in ways that were undreamed of by our alchemistic predecessors.They also represent the merger and the mutual confirmationof the Newtonian paradigm and the atomic theory.The concepts of energy and momentum, for example,are applied to orbiting electrons which are attractedto a nucleus like the solar system by an inverse square forcewhich has the same form as gravity, but which operateson the quality that we know as charge, rather than the quality we know as mass.Admittedly in understanding these concepts, new principlesare involved; namely, those principles of the quantumtheory--those strange theories of the quantum of radiation.But as it turns out, the quantum principle is the missing linkbetween the Newtonian world of force and energy,and the atomic world of chemical reactions.

The 19th century was a period of remarkable growthin the sciences, not just in chemistry, but in all the sciences.In fact, from the discovery of the electrolysis of water to atomictheory, to the periodic table in chemistry, to conservationof energy, and electromagnetic theory,and atomic particles and kinetic theory in physics.In chemistry, at the beginning of the century,people still believed in caloric theory.Lavoisier had just come up with his theory of the elements.The battery was invented, and the laws of chemistryand atomic theory were put forth.By mid-century, kinetic theory, conservation of energy,the study of electromagnetism, physical chemistry;things like reaction rates and energy and electrochemistry,inorganic chemistry were fully accepted, and also the theoryof evolution, Darwin's theory, came into beingin the mid-19th century as well.By the end of the century chemists had isolated the periodic table,discovered the electron, radioactivity and x-rays,and were setting up the next stage which would be the quantumtheory and the understanding of the atom thatwe reviewed in a previous program.So, these changes took place so fast and there were so manypeople involved that we can't really detail all of the changes and all the people.So, what we want to try to do in this program is to summarizethese major events which led to the understandingof the periodic table and the development of the chemical theory of bonding.

The growth of the science of chemistry as far as thediscovery of new elements goes is really well exemplifiedby the concept of the "rare earths."In the 19th century the word, "earth," meant an insoluableoxide of which did not compose when heated.These were usually oxides of metals and they were very commonly ores.We've talked about this before that also the term, calcs, had been used.Now, "earths" were not uncommon and many of the new elementswhich had been discovered by electrolysis in the earlyand middle part of the century were extractedfrom their oxides or from their "earths."

In 1794, a chemist named Johan Gadolin was shown an "earth"from a quarry in Ytterby in Sweden, and its propertieswere unlike any of the known "earths" at that time.This became known as a "rare earth" when itsproperties were described.The chemists suspected that it contained a new elementbecause it had properties unlike any of the known "earths."In 1803, two chemists discovered the element named Cerium,which is named after the asteroid Ceres, which hadbeen discovered the previous year.

In 1839, 35 years later, Mosander studied Gadolin's "rare earths"and concluded that they contained a mixture of new elements,not just a single element, but a mixture of newelements all with very similar properties.He isolated a chemical he called Lanthanum which is the Greekword for hidden, and later in the century, spectroscopyand the periodic table helped to discover other "rare earths."The question here, why we mention the "rare earths"in the first place, is that the properties of the variouselements that make up the "rare earths" are much more alikeeach other than they are like any other elements.In fact, this is why they were so hard to isolate, and so hard to discover.Simply because the elements are so much like one of the other.So, the question comes up then, "Why should these elements,unlike all other elements have these properties that areso much alike, and why do they have some of the properties?"What is it about these elements that causes them to be so much alike?

Now keep in mind that as far as the properties of the elementsgo, nobody had done much work on this.Lavoisier's listed elements which he published in 1789in his treatise on chemistry, "The Textbook," was merely a listing.He did not include any descriptionof the properties of the elements.He gave the names.He described what they were good for, but didn't list the properties.Many new elements had been discovered by the middleof the 19th century, and their properties had not been reported.So, I want to refresh your memory about physicalversus chemical properties for a minute.Physical properties are things like hardness and shininess,and malleability and density and conductivity.The chemical properties of substances, elements, are methods of preparation.How do you prepare them?Whether they're soluble in acids.How do they react with other elements?In what proportions do they react with other elements?Is the reaction fast or slow?All these kinds of things were beginning to become wellunderstood in the early part of the 19th century, and by the middlepart of the century had become fairly well known.The other physical property, part of the element, which was thehardest to get a handle on, was the concept of atomic weight.We mentioned earlier how Avogadro's method gaveus a way to estimate atomic weights.

In the middle of the 19th century the methods of determiningatomic weights were perfected, and most substances werecomposed of atoms which had fractional atomic weights.For example, the element, chlorine, was found to have an atomic weight of 35.5.Now, this doesn't seem to make sense if the atoms are basedupon some sort of a fundamental unit like hydrogen, for example.How could you have an atom with a fundamental weight which was a fraction.It turns out that later on the mass spectrograph showed that thesewere naturally occurring isotopes and that chlorine actuallyconsists of about 50% chlorine 35, and about 50% chlorine 36.But, I'm getting ahead of the story again, so I don't want to get too far ahead.So now we want to turn our attention to the periodic tableand the patterns of chemical and physical properties.This began a long time ago actually, the idea of patternsof properties, and we've seen some of thiswith the alchemists looking at, for example, different metalshaving similar properties, and van Helmont subjecting the metalsto the saint acid test and that sort of thing.But the first serious attempt at this began back in 1817,and this was the theory of triads.

A German chemist named Johann Dobereirner noted thatthe elements could be grouped in three, in different waysaccording to the chemical properties, in groups of threes.He noticed, for example, that the atomic weights of certaingroups of elements were almost the same.A good example of this is the elements, iron, cobalt and nickel.All have atomic weights around 55, 56, 57.And in many substances the middle atomic weightwas about the average of the other two.For example, three substances, chlorine, bromine and iodine,have, if you average the atomic weights, they are exactly equalthe atomic weight of bromine, which is in the center of them.This, as far as anybody knows, was the first real attemptto group the chemical elements by any kind of reasonable properties.Nothing much happened on this until the late 1800s whenin 1864, an Englishman named John Newlands grouped theelements in order of their atomic properties, atomic weights, I should say.And he noted, when he did this, that every eighthelement had very similar properties.In fact, Newlands went before the Philosophical Society of Londonand presented a paper of this, in which he stated, "The eighththe element, starting from a given one, is a kind of repetitionof the first, like the eighth note in an octave of music."He called this the law of octaves.It's interesting that he was laughed at by the otherscientists at the Philosophical Society because it seemedtoo musical and too Pythagorean.In fact, one of the scientists present at that meeting askedif he had ever tried classifying the elements in the orderof the initial letters of their names, assuming that,you know, he was just randomly doing this.

I should point out that he was later exonerated for this viewand was awarded the Humphry Davy Prize which wassimilar to the Nobel Prize at that time.You see what's happening here, though, the idea that this wasrelated to music and was simply related to the octet, was too Pythagorean.By this time, the idea of the Pythagoreans that things couldhave meaning just because they were numericalpatterns, were scoffed at by scientists.So, the real periodic table, the one we're familiar with today,was put together simultaneously,but independently, by two different chemists.One of these was a German named Lothar Meyer,and the other was a Russian named Dmitri Menedleev.They didn't work together, and, in fact, they sort of overlapped.Meyer actually published first and then Menedleev published,and then Meyer published more, and by the time everything gottogether, it was realized that there were twodifferent things going on here.

The periodic table, as we know it today, represented a significantand tremendous leap in the understanding of matter.It provides several different things for us.One thing, it's a basis for understanding chemicalreactions and properties, and it strongly supports atomictheory for reasons that we'll see later.And it also suggested that elements were composedof similar building blocks of some kind.This is even before the discovery of the electron.And it also suggested that there was somehow some smaller patternstructure within the atom which controlled the chemical properties.So, Meyer's table was based upon physical properties,whereas, Menedleev's was based on chemical properties.So, since it's basically Menedleev's table that we usetoday, let's focus on Menedleev's table. Menedleev, himself, was a very interesting characterand he deserves a little bit of a biographical sketch.If you had to relate the ultimate Russian tragedy,Menedleev would fit into that category.He was born in Siberia, the fourteenth child of a poor teacher.His father became blind and he was cared forby his mother who managed a glass factory.

In 1848, she recognized that Dmitri had a certain amountof intelligence and so she took him and traveled by foot across Asiato enter him in the University of Moscow.Now, I want to point out to you, this would be equivalent totaking a trip from New York to Los Angeles on footin the middle of the Siberian winter.If you've ever seen the movie, "Dr. Zhivago," you get an idea.I can picture them eating cold potatoes and drinkingvodka and stuff along the way.He gets there to the University of Moscow and he's denied entrybecause he's a Siberian and nobody knew, they didn't knowbeforehand that they didn't allow Siberians into the University.So, they left Moscow for St. Petersburg, which is another 500 miles away.And finally in 1850, two years after they began,he was admitted to a training school for teachers.And once they got to St. Petersburg, his mother died in that same year.What a tragic story.I mean talk about the hardships, and talkabout having to make it on your own.Most of us today could learn a lot from this, I think.He eventually worked his way up and became a professorof chemistry at St. Petersburg, where he eventually resignedin 1890 over a dispute with the authorities over his viewswhich were considered to be too liberal.Where have we heard that before in the scientific world?He wasn't chastised the way Galileo was, but he wasbasically forced to resign because his views on chemistryand social things were considered to be too liberal.And I point out this was before was before the Bolshevik Revolution.

Menedleev died in 1906, so it was a long time before the Russian Revolution.So, what he did basically was to publish a periodictable based upon the chemical propertiesof the elements, not the physical properties as Meyer had done.And he also, then, discovered or suggested,I should say, undiscovered elements.People were skeptical of this sort of pattern thingafter the attempts of, the earlier attempts and the Pythagoreannature of this patterns, but, one reason why Meyer'stable wasn't accepted so readily.

Menedleev predicted the location of new elements because theatomic weights seemed wrong and he sometimes even, where theatomic weights didn't seem right in the pattern, he's switch themaround to make the atomic fit into the pattern better.He left blank spaces in the table and predicted the propertiesof missing elements which he called eka boron, eka aluminum and eka silicon.These were elements which would seem to fit into this schemeof eight, this pattern of eight, which we'll talk about in a minute.These three soon were discovered and they hadproperties very nearly as predicted.

Properties including the melting point and the densityof the solid and the reaction with certain chemicals and the waythey dissolved in other chemicals and things like that.So, compared with the modern periodic table, there were a lotof gaps because Menedleev only knew of 60 elements.We'd know today of 92, plus the transuranian or artificial elements.And the status of some of those elements wasuncertain in Menedleev's time.So, his chart was arranged by atomic weight notby atomic number the way we do it today.And it was also based upon chemical propertiesunlike Meyer who based his mainly upon the density of materialsand never did get around to making the prediction.

Meyer didn't have the confidence that Menedleev had to make theprediction about the existence of the new elements.Now it's time to take a look at the modern periodic table and seehow it explains some of the things or how it organizes someof the aspects that we know about substances.The first thing we want to look at is the distinctionbetween metals and nonmetals.If we look at the modern periodic table, we seeon the right hand side this stair step line.The stair step line separates those substances which havemetallic properties from those substances which have nonmetallic properties.Everything to the left of the stair step line is, in fact, metallicand everything to the right is, in fact, nonmetallic.Although we'll see in a minute that things that lieon the borderline here like aluminum and silicon,boron and carbon, and so forth, have properties that aresort of semi metallic and semi nonmetallic.So, what exactly distinguishes a metal from a nonmetal?Well, metals are mostly solid at room temperature,except for mercury which we know is a liquid.They're shiny.They're hard.They're malleable, meaning that they can be hammeredor drawn into thin sheets or into wires, and they're for the mostpart, good conductors of heat and electricity.

Nonmetals, on the other hand, may be solid, liquid or gas.Things like oxygen, nitrogen, carbon, fluorine, chlorine,bromine, all the things on the right hand side of the periodic table.You may find it useful, by the way, to look at the periodic tablein your textbook while we're referring to this section.The solids, which are nonmetals, are usuallyhard and brittle, rather than malleable.And they're generally not good conductors of electricity.The only exception is the graphite form of carbon.And I mentioned earlier that elements near the division linemay have some properties of both and I do have some exampleshere of some of these elements that are right near the line.We can take a look at this.So here, for example, is a piece of silicon.Now this is a silicon, crystalline piece of silicon.It doesn't occur naturally, but this is derived.This is the kind of stuff that they make computer chips out of,and next to it is a piece of carbon in the form of graphite.Notice how they look very similar.They have a shiny appearance, but they'renot malleable like the aluminum.This is a piece of aluminum foil that's balled up.And the aluminum, of course, is capable of being putinto a flat sheet because of its malleability.Over here is a piece of boron.

Boron, again, doesn't occur in a natural form in nature,but notice that it's similar to the carbon in the silicon,except that it doesn't look quite as shiny and metallic.So, these are things which have properties which are semi metallic.I pointed out before that the graphite form of carbonis a good conductor of electricity, and it's oneof the few nonmetals which is a good conductor.Aluminum, of course, is a decent conductorof electricity and it's used, in fact, in wiring.Boron doesn't conduct electricity at all,and silicon is what we know as a semi conductor.And that, of course is what makes it useful in computer chips.In the periodic table if we look at the next, right below silicon,if you look at your periodic table, you'll see silicon, and below it is germanium.You'll notice that the two of these are in the same column.This is not just a coincidence.

The periodic table consists of columns and rows which reflectthe organization of the table and also reflect the periodicityof the chemical properties that we talked about earlier.Now we can take a look at the rows and columnsof the periodic table and see how they reflect this organization.So, going back to the periodic table.The first thing we notice here is that the things in the variouscolumns have the similar chemical properties.

Now when we say, similar chemical properties,what we mean is simply that they will combine with the sameamount of atoms of hydrogen or oxygen.For example, hydrogen forms a compound with water called H2O.With oxygen, I'm sorry, called H2O.Lithium, forms a compound with oxygen called Li20, lithium, lithium oxide.Sodium forms a compound with oxygen called Na20.On the other hand, in the next column beryllium, magnesium,calcium, strontium, barium, all form substances in a one to one ratio with oxygen.So, the substance would be MgO, CaO, SrO, and BaO and so on.So, what we see is that as you go down the columnsof the periodic table, everything in the same column have similarproperties, even the things that cross the metal,nonmetal line like boron and aluminum.And as I mentioned before, silicon and germanium have similarproperties, even though one's a metal and one's a nonmetal.On the other hand going across the rows of the periodic table,starting with hydrogen going to helium, back to lithium,beryllium, boron, carbon, nitrogen, oxygen, fluorine, and so on,we find increasing atomic weight and we find also increasing atomic number.In fact, the atomic number's nothing more than the addressof the substance in the periodic table.

The other thing we note then is the groupings of theseelements in the periodic table.Notice there are two different kinds of groups here.If you look at your own periodic table, you can't see this verywell on the screen, but you'll notice that the columnson the outside here are labeled 1A, 2A, 3A, 4A, 5A, 6A, 7A, 8A.The ones in the middle are labeled with B's instead of As.There's some significance as to why we have A groupings and Bgroupings, and we, I think we'll have to come back to that alittle bit later when we get into what the periodic table reflectsin terms of the electron structure of the various atoms and elements.The nobel gases represent the completion of the modern periodic table.You see Menedleev's periodic table was missing a whole columnof gases because they were undiscovered in his time.They're called the noble gases or sometimes the inert gasesor even the rare gases or sometimes even called the helium group.They went undiscovered because they do not form compounds.They're noble, meaning that they don't react chemically with other substances.Because these are all gases, and because they're not particularlyabundant, they were hidden from the chemical observationby the scientific community for two centuries.And all of these gases do occur in the atmosphere to some degree.

The first clue to the existence of the noble gases was ignoredby the scientific community, and they were discovered, actually, by Cavendish.You remember Cavendish, who weighed the earth and discovered hydrogen.What he did was to remove nitrogen and hydrogenfrom the air and he did this by continually sparking electricspark which caused the nitrogen and oxygen to react as all deadin water, dissolve the carbon dioxide in water and when hedid all this, he found that there was still a tiny volume of air left.It amounted to about 1/120 of the volume of the original air.That's about 8/10 of a percent.

Most experimenters, even in modern times, would haveignored this and simply put it off as to experimental error.You know, close enough is close enough.But Cavendish was concerned about this.And his experiment was repeated in 1894 by Raleigh and Ramseyand they now used the spectroscope.Remember the spectroscope?To recognize a new element in this gas which they named argon,which is the Greek word for inert.A little bit earlier, in 1868, in fact, a man namedPierre Janssen, and don't worry about remembering all thesenames, in 1868, observed the solar spectrum with the spectroscope.And here we begin to see why the spectroscopeand why spectroscopy is so important in all this.What he found was a bright yellow line in the solar spectrum whichwas not attributed to any known element.He didn't know what it was and later on a British astronomernamed Lockyer suggested the element was not found on earth at all.So he named it for the sun from the Greek word, helios, and he called it helium.Later on when Rayleigh and Ramsey found argon,they recognized this as a whole family and they eventuallyisolated helium on earth and also found the other noble gases thatwe now know as krypton, neon and xenon.The most recent of these was the radioactive noble gas calledradon, which was discovered by the Curies as the daughterof the uranium decay in radioactivity.

So, if we go back to the periodic table now, we can see howthe noble gases fit into the scheme.Over here on the right hand side, we see this entire familyof the noble gases, helium, neon, argon, krypton, xenon and radon.No, this is not the krypton as in kryptonite, Superman is not afraid of this one.It's simply a noble gas.Many of these we've already looked at.Helium we know about.Neon we seen the light on the emission lamp.The noble gases will turn out to have a central rolein understanding the relationship of chemical bonding which we'llget to after we do the electron structure of the atoms.

Now we're ready to come back and examine the electron patternsand the role of electrons in chemical bonding.But first, we sort of need to review or capitulate or putourselves in perspective of what all this means in where we are.So, here's what's happened.The atomic theory spurred the progress of chemistry beginningin the early 1900s from the trial and error of the alchemistto a very systematic discovery of formulations.There were many generalizations and creations of compounds,and ideas about what works and what doesn't,but even so, at the beginning of the 20th century there weremany unanswered questions about chemistry.For example, why do the elements have the properties that they do?In other words, why is the periodic table arranged the way it does?And why do some elements occur as diatomic pairs, and others assingle atoms and others solids of uncertainformulas, things like carbon and sulfur?Why does hydrogen combine with oxygen in a two to one ratio?It's been known since early the 1800s that it did this, but why?And why are some compounds soft with low melting temperaturesand others are hard with high melting temperatures.?And why do some substances dissolve in water and some don't?And why are some substances electrolytes?In other words, why do some substances conductelectricity while others don't?

The answers to all these questions came out of the atomic theory.And this is a perfect example of the fact that many times a newtheory provides more questions than it answers,and this is a perfect example of this.The answers will come from the physics of the atom.Remember, on the physics side, we were discovering pieces of atoms.We had cathode rays.We had radioactivity.We found the charge and mass of the electronfrom Millikan's oil drop experiment.Rutherford with gold foil and the atomic bulletsdiscovered that the atom is mostly empty space.At the same time the photoelectric effect shows usthat the light waves had particle like properties and DeBroglieshowed us that the electron, a particle, has wave likeproperties, and all of this comes about from the ideaof spectroscopy which not only allowed new elements to bediscovered, but which also allowed the discoveryof the properties of the electron and the atom.Later on in the middle of the 19th century the mass spectrographshowed the existence of isotopes.

A mass spectrograph is simply a device which allows atomsto be sorted according to their atomic weight, very precisely.Isotopes are simply the nucleus of atomsof the same elementwhich have differentweights, violating oneof Dalton's atomic principles.So, this all leads to a whole lot of new things coming up.And I want to point out that chemists were very quickto adopt the nuclear model to explain their discoveriesand to make new predictions, and this really represents theultimate marriage of the physical sciences of physics and chemistry.Before we go on and learn about the structure of the electronsaround the atom, we need to briefly review this nuclearmodel that we came up with in a previous program.

Atoms, remember, consist of a dense nucleus of positive chargesurrounded by a cloud of negative charge.We didn't study the details of the structure of the atom,but I'm sure you've heard this before that the positive chargesin the nucleus are called protons.They carry positive charges in the same amount of charge,the same strength of charge, as the negative electrons.So they're opposite in charge, but the proton is much more massive.In fact, the proton weighs about 1,800 times as much as an electron.So, the atom can be thought of sort of like a papayawith a cloud of fruit flies buzzing around it.OK?The papaya being much more massive than the fruit flies.

The electrons occupy certain energy levelsaround the nucleus and these define the outer boundaries of the atom.So, the electrons also present a shield which prevent the nucleiof atoms from touching each other, and we will see that itis these electrons, specifically, the electrons in the highestor outer most energy level which are responsible for the chemicalproperties of the elements and responsible for chemical bonding.We can get a hint to some of the answers to these questionsby looking at these patterns in the electron structures.The patterns, in fact, have to do with the atomicweights of the noble gases.In fact, if we look at the noble gases, their atomicnumbers differ by certain amounts.

I've listed here the atomic numbers of all the noble gases,helium, neon, argon, krypton, xenon and radon.Look at the numbers here, 2, 10, 18, 36, 54, 86.Now this is a series of numbers, right?But what we want to look at is the difference between these numbers.For example, the first number is 2.The difference between 10 and 2 is 8.The difference between 18 and 10 is 8.The difference between 18 and 36 is 18.18, 36.The difference between 54 and 36 is 18.What's the difference between 54 and 86, quick, do your arithmetic.It's 32.So, what?What am I saying here?I'm saying that there are certain magic numbers that appear overand over again here, the numbers 2, 8, 18, and 32.Let's go back to the other slide.Two, 10 minus 2 is 8; 18 minus 10 is 8.So, here's the number 2 and 8.Then the number 32 crops up because, I'm sorry, 18 crops up,because 18 is here, but 18 plus 18 is 36, and 36 plus 18 is 54.And the next number, 32, shows up in the fact that 54 plus 86 is 32.So, what does all this mean?What it all means is that the answer lies in quantum mechanics.

The quantum mechanics has something to do with the shellsof electrons and the way they electrons are arranged around the atom.Now I'm going to go into more detail about this in a couple of minutes.But, basically it works like this.The shells of electrons, remember the, that each, there aredifferent layers or different levels of electronsin the atom, each level of the electron can contain sublevels.It's sort of what you would get, you know, looking at a stadiumwhere you have the red seats and the blue seats and the orange seats.Each one represents a level, but there are sublevelswithin the rows of those seats.So it turns out that each shell, is what the layers are called,and each subshell, which is what the sublayers are called,can contain a fixed number of electrons.And it turns out that when you start with the lighter elements,like hydrogen, and work your way up the periodic table throughhelium, through lithium, through beryllium and so on, that thefilling of the shells and subshells of the electron orbits,take place in a very orderly and structured way.

If we go back and look at the periodic table, this shows veryclearly in the periodic table this arrangement.So, here's the periodic table.Notice here that on the left hand side, there are two columns.There's the number 2.In the middle there are 10 columns.That represents the number 10.Here there are six columns.Well, the number 6 didn't show up, or did it?Well, look at the patterns up here.The first column, the first row, I should say,has 2 elements, the second column has 2 elements,the third section over here has 6 columns--2 plus 6 equals 8.So, what's, if you look at the difference between the atomicweight of helium and the atomic weight of neon, they differby the number 8, because there's 2 plus 6.With me so far?OK.

Now, this middle section contains 10 columns.Ten columns, so what's the difference in weightbetween neon and the next element, argon?Well, it's still 8, because these columns don'tappear yet in this part of the periodic table.So, here we have the difference between helium and neon is 8.The difference between neon and argon is 8.That reflects the 2 plus 6; and then, the differencebetween argon, now, and krypton is 2 plus 10 plus 6.Which is 18.Is this all making any sense?Now, where does the 32 come in?Well, notice that there's this subcolumn down here.This contains the "rare earths."Remember the "rare earths?"They were a bunch of chemical elements which all had similar properties.It turns out that the "rare earths" all fitinto the same column of the periodic table.It's as if this particular column has two moresubgroupings which contain 14 columns.One of these is called the lanthanides, or "rare earths."The other is called the actinides which includesuranium, actinium and that sort of thing.So, here's where the number 3, 2 comes in.Now 2 plus 6, plus 10 plus the extra 14 columns in here adds up to the number 32.

Now, here's the question we've got to ask.Do these numbers mean something?Or are these numbers simply Pythagorean coincidences?Remember that Pythagorean coincidences were things thatscientists had looked to ignore by this point and that the originalsuggestion that the periodic table might reflect some sortof numbering, was rejected exactly for this reason.It turns out that the reason for all this is that theplanetary model of electrons is incomplete.Electrons in orbits are not same thing as satellites around a sun.That is because they can only be in certain shells, the quantitized.Right?And each electron shell has a limited number of quantum substates.I don't want to get into the details of this, because it becomes verycomplicated, and basically it works like this.There, the properties of electron are such that the electrons arespinning, they're moving around the atom in unpredictable ways.But when the atoms spin and move around they createelectric and magnetic fields.And it turns out that the properties of these fields aresuch that the electrons can only exist in the atom in certain ways.So, there are definite maximums of numbers of electronsin a given energy level and these are defined by mathematicalrelationships, which we don't want to go into.

Now the problem here is that the electron orbits,themselves, cannot be known precisely.This has to do with the wave nature of electrons.When you start getting down and looking too closely at electrons,their wave properties take over and you can't tell exactly where they are.But what we know is that the electrons are in differentenergy levels, which we talked about before in the Bohr atom,which we just now referred to as shells.So, what does all this mean?Let's summarize it this way.

By noting that the energy of the electron is the sumof the kinetic and potential energies, and the momentumof the electron is both angular as electrons spins and movesaround the atom like, and also, spin as the electron, itself, spins like this.So, when you take all these things together, the kineticand potential energy, the angular momentum and the spinof the electron, the electrons can settle into certain stableconfigurations in the atom, and the shells, then, consistof subshells, the number of which get greater, as the principlequantum number or the shell number gets greater.In fact, these numbers, the 2, 8, 18, and 32 can be expressedvery nicely by a very simple formula.If the number "N" equals the number of the shell,then the maximum number of electrons per shell is simply 2N2, two "N" squared.What a nice simple formula.And if you apply this, if you look at the numbers in the columns,2, 8, 18 and 32, you'll find that they correspond to "N" equals1, 2, 3, and 4, with the formula 2N2.

It turns out now, and we'll come back and dealwith this in a couple of minutes that the moststable arrangement of electrons is anarrangement which has 8 electrons in theoutermost subshell--8 electrons in the outermostsubshell.So think of the different shells as different layerswithin this, within each energy level.And it turns out that it's not just 8 electrons, it's 4 pairs of electrons.It turns out it's the pairing of electrons that's really important.To simplify our understanding of the relationshipbetween the periodic table and chemical bonding,we want to look simply at what we'll call the representativeelements and the transition elements.If we look at the periodic table, the 2 columns on the leftand the 6 columns on the right are what we call the representative elements.And these all have group numbers that are numbered "A."I mentioned this before the "A" and "B" groups.

The number of electrons in the outermost shell,the valence electrons as they're called in theseelements is the same as the group number.So those elements in group one have 1 electron.Those elements in group two have 2 electrons in the outermost.Those in group three have 3 electrons, and so forth,all the way over to the noble gases which have a fullshell of 8 valence electrons.Remember that 8 is the maximum number that you can have in any shell.Now, the things in the middle here are called the transition elements.They have "B" numbers on their groups.And the number of outer electrons here is much more complicated.This complication is due to the way in which the sublevels fill,and the difference in energy levels between the various sublevels.It also, I should point out, is responsible for the propertiesof these transition metals, which we'll talk about a little bitlater on in terms of their metallic bonding and covalent bonding.So, we'll ignore basically the transition elementsin this upcoming discussion because once we knowfor sure what the outer number of electrons is,based upon the group "A" numbers, then we can easily understandthe chemical properties and predict why H2O is the formulafor water instead of HO and those other questions thatwe asked a little bit earlier on.

Now we're ready to take a look at the real reason for chemicalbonding which has to do with these electron shellswhich are exemplified by the periodic table.The chemical bond is caused by electrical forces withinand between atoms and also by the nature of electrons whichdetermines their arrangement around the atoms.So, what we have here is a magic number.The magic number is 8.These electrons tend to occur in 4 pairs, so which makes8 electrons in the valence shell or the outer shell,except for the first shell which only has 2 electrons.We don't want to get into exactly why that is, but again,it has to do with the limited number of quantum statesavailable in the first shell.So, the inert gases, as you recall, all have a magic number of electrons.In other words, the inert gases are inert because they're allin group 8 and they all contain 8 electrons in their outermost shell.This, by the way, is called the "rule of octets."

The word, octet, meaning 8.The exception, of course, is hydrogen and helium,because helium has only 2 electrons in its outer shell,but since that's the first shell, it represents a filled shell.So, the neutral inert gases have stable electron configurations.And I should point out here that these are not totally inert.That they have been made to engage in chemical reactions,but you have to work really hard to strip the electrons away from them.So, here's the secret now behind chemical bonding.Other electrons can get a magic number of electron eitherby gaining or losing electrons to become ions.In fact, 3 or less electrons can be gained or lost from the valenceshell to give each atom a, what we might call,a noble gas or magic number configuration.

Let's go to the computer and I think we can see what this looks like.So, if we rearrange the periodic table and put the noble gasesin the middle, we note that they all have the magic octet;that is, they have a complete shell of electrons.On one hand, the nonmetals tend to gain electrons and can gainelectrons by, and become the, have the configuration of the magic octet.For example, those things here in group VII have 7 electronsin their outer shell and they need only 1 electronto become a, have a full shell.You might think of it this way.There are two things going on here with each atom.On one hand there's a tendency to balance thepositive and negative charges.On the other hand there's the tendency to fill the electron shells.So, the huygens, as they're called in group VII, all can gain oneelectron, have a full outer shell, and wind up as an ion,a charged particle, with a charge of minus one.Those things in group VI can gain 2 electrons.

Notice the difference between VI and VIII, is minus 2.If you subtract VI from VIII you get minus 2.So, on the other hand, on the other side, are the metals.The metals are those substances which have 3or less electrons in their outermost shells.So that, sodium, for example, which has only one electronin its outer shell can lose a single electron and attain the magicoctet and when it does so, well, let's take a look here, what would happen?It loses an electron.That means it leave behind extra positive charges in the nucleus,so it winds up as, with a charge of one.So, let's look at the chemical reaction between sodiumand chlorine to form halite or table salt.Sodium has 1 electron that it loses very easily.Chlorine, on the other hand, likes to gain the single electron.What a suitable arrangement for them.So, sodium loses an electron, the chlorine picks up the electron,the sodium becomes a positively charged ion;the chlorine becomes a negatively charged ion.So now imagine a pile of ions lying around.You take a bunch of sodium and a bunch of chlorine, electronsare transferred, or stripped of the sodium,given to the chlorine, so now you have a pile of billionsand billions of ions of sodium and chlorine.What happens to them?Well, what happens to them is that they stick together becauseof their opposite charges, and form what we call an ionic compound.

Now, I want to note here that this is not the only type of chemical bonding.In fact, that pairs of electrons are favored over single electrons,and that this is not the only way to do the bonding, and we'll seehow this works when we take a look at the types of chemical bonds.Now we're ready to look at the various types of chemical bonds.They're actually four different kinds of chemical bondswe want to look at, and I've included something calledphysical bonds here in the description, and even thoughthey're really physical, we'd consider them in chemicalbonds because they involve chemical type processes.So the first of these is the ionic bond.We've already learned about this.This is the type of bond where there's an electrontransfer from one atom to another.This usually occurs between metals and nonmetals.And this always occurs in all of the groups but group IV.

The atoms in group IV simply do not form these types of bonds.In fact, the atoms in group IV tend to form covalent bonds.The reason why is because these group IV atoms have 4 electronsin their valence shell, and 4 electrons is simply too muchto strip off and it's too hard to stick 4 more electrons onfor them to have a completed shell.So what this is, is basically a sharing of electrons,instead of a stripping of electrons to complete the octet.So what happens is that it's the unpaired electrons thatare shared between atoms and it's actually more likely that thesewill form between atoms of similar properties.Now I should also point out that the more similar the propertiesof the two atoms are, the more likely it is that the electronsharing is equal, like, for example, between two atoms of the samekind, like oxygen or nitrogen, or hydrogen, or the other diatomic gases.The sharing sometimes is unequal and when this happens,we get what's called a polar molecule where oneof the atoms takes more of the electron than the otherand these polar atoms have interesting effects,which we'll come back and look at when we look at hydrogen bonds.To see how this works, a chemist named Lewis invented a wayof visualizing the covalent bond through what'sknown as today as Lewis pictures.

So let's go to the ELMO and see what Lewis pictures look like.The Lewis pictures involved the fact that all of the elementsin a given column have the same number of electrons in their outermost shell.So, for example, we could note the atom, lithium, and we could lookat lithium as simply having one electron in the outer shelland portray this with one "X" which indicates that electron.We can look at sodium, the next element in that column oneof the periodic table, and it also has one electronin the outermost shell, so we can portray sodium also with a single electron.If you look at the periodic table again, you'll see that at the topof this column is hydrogen which also has a single electron.So we can portray all of these things in column onewith a single electron in the outermost shell.This doesn't reflect how many total electrons,but it's only these outermost surveillanceelectrons that are involved.So, let's look at the reaction between hydrogen and oxygen.Oxygen occurs over here.It's in group VI, and so oxygen will have 6 electrons in its outermost shell.

Now, the way the pairing thing works is that theelectrons are only paired if they must be.So, the electrons in oxygen are arranged around theoxygen atom can be represented this way.In other words, there are two pairs of electronsand two unpaired electrons for a total of 6.Now, if we look at this from a larger scale, we can lookat a hydrogen atom over here with one electron and what happenswhen you put that in the presence of an oxygen which has a totalof 6 electrons, two of which are unpaired.

Now you can see immediately what hasto happen to complete the octet, can't you?The oxygen needs to have 2 electrons, the hydrogenhas one electron, so how does this work?It's really very simple.In the bonding, the one electron between the hydrogenand oxygen is shared between the two of them, so that basicallythe hydrogen atom moves over next to the oxygen atom,and it takes 2 hydrogen atoms to complete the octet, because theoxygen atom needs 2 electrons, so this electron will move overhere to pair with this unpaired electron.This will move over here to pair with this unpaired electron,and the atom as a whole, I should say the moleculeof water as a whole, now looks like this.Here's the oxygen.Here's another hydrogen down here,and when you look at the arrangement,each of the atoms now thinks that it has a full complement of electrons.What I mean by that is the hydrogen looks at its electronsand says, "Hey, I've got 2 electrons, I've got acompleted outer shell, I'm happy."The other hydrogen looks and says, "Hey, I've got a completedouter shell of electrons, I'm happy."

The oxygen atom at the same time looks at its completed shellof 8 electrons and says, "Hey, I've got a completedshell of electrons, I'm happy, too."So, it turns out that for any kind of covalent bonding,we can explain why a certain number of atoms of one kindcombine with a certain number of another atoms simply by lookingat this position in the periodic table, the valence electronsand the number of unpaired electrons involved.So the next type of chemical bond we want to consider is the metallic bond.The metallic bond, of course, forms between the atoms of metals.This is a difficult one to understand, but basicallythe metals, these are mostly the transition metals,but also the alkaline metals as well form from delocalized electrons.

Delocalized electrons means that basically the electrons are freeto wander around from atom to atom and you can sort of thinkof this as a bunch of positively charged ions floating in a seaof negatively charged negatively chargedelectrons in a metallic crystal lattice.By this crystal lattice, of course, we mean this orderlyarrangement of the solids that we talked about in an earlier program.The strong material such as iron also have covalent bonds.And this explains why the transition metals are strongmetals, whereas, the alkaline metals like sodium are weak metals.And you may remember in a previous program I cut apiece of sodium with a table knife.

The sodium contains only very weak metallic bonds,so that the, it's very easy to cut.This sea of electrons, by the way, also explains the electricalconductivity and the heat conductivity of metalsbecause the electrons are basically bumpinginto each other and are free to transmit energy likethe swinging balls as they bump into each other.OK.The last type of bond we're going to talk about here are the physical bonds.Now, these are called physical bonds,but they're still due to the same sorts of features.These are weak bonds which are due to unbalancedelectrical forces in the atoms themselves.These are basically of two types.One of these is called van der Waals bonds.It's these type of bonds that hold together the layers of graphiteand hold together the layers of the minerals that we call mica.These have to do with the electrons as they spin around,they sort of leave behind a magnetic residue which attracts other atoms.The other type is probably more important.These are called hydrogen bonds and these have to dowith the fact that when hydrogen and oxygen combine,the oxygen steals away the electron more than the hydrogen does.In other words, there's an uneven sharing.What this means is the electron spends more timearound the oxygen atom and less time around the hydrogen atomso that the hydrogen end of the molecule is slightly positivelycharged and the oxygen end is slightly negatively charged.

In water, for example, the water molecules tend to have aslightly positive charge on one end and a negative chargeon the other end, which causes them to be sticky,so that the positive end of one water molecule is attractedto the negative end of another water molecule, and it's thishydrogen bonding that explains all of the properties of water likethe high latent heat and the high specific heat, the formationof ice crystals, and this various types of physical things thatwe talked about in earlier programs.So hydrogen bonds are very important, and we don't havetime to go into more detail in this, but you'll find them importantfor things like curling hair, for holding water together,and for coiling proteins to make them into the right shapes,for enzymes and lots of other different things.Now we're ready to apply some of this to understand the processof solutions, electrolytes, acids, bases and salts.The first one I want to look at is the conductivity of water.It's very clear that pure water does not conduct electricity.

We normally think of water as being a good conductor,but that's due to the impurities in it.But certain substances do increase the conductivity in water when dissolved.Things like sugar and alcohol do not increasethe conductivity, but things like salt do.So we can define something called an electrolyte which is asubstance which increases the electrical conductivity of water.Now it's interesting that dry salts don't conduct electricity either.And the reason for all this is that salts are ionic substancesand they simply have free charges to move within the solution of the water.

Acids are substances which turn litmus red.We've all heard the word, acid, before and we knowthat acids are corrosive to things like metals.What we don't know, or many of us, is that acids resultfrom the water solution of the oxides of nonmetals,things like sulfur and nitrogen and carbon.Acids have a sour taste and they react with bases to form salts.What's responsible for acids is that the acids releasehydrogen ions into the water.So, something which is an acid is something whichcontains hydrogen ions in solution.And the hydrogen ion may attach itself to a water moleculeto form a hydronium ion or it may be free as anindividual hydrogen ion stripped of its electrons.Strong acids are those substances whichcompletely yield their hydrogen ions to the solution.The solution reaction, you might say, recedes to completion,so nearly all the hydrogen contained in the compound is released as ions.Examples of this are things like hydrochloric acid, HCl,which is an ionically bonded substance.Another one would be hydrofluoric acid, HF.So, by contrast, weak acids are weak electrolytes which yieldonly some of their hydrogen atoms as ions.In other words, the solution does not completely stripthe hydrogen atoms away, and some hydrogen remains bonded to the molecule.

Many organic acids like vinegar, acidic acid, is like this.And so what happens basically is that the hydrogen,some of it remains attached, and some of it remains free.Bases are also called alkalis.This is another category of substances that sortof operate in contrast to acids.They turn litmus blue.They have a bitter taste and they have a slippery feel.Things like soap are good examples of a base.Now, the bases result from the aqueous solution of the oxidesof metals, as opposed to acids which are generatedfrom the oxides of nonmetals.So, generally, it's the metals nearer the left sideof the periodic table form the strongest bases.Things like sodium hydroxide and potassium hydroxide.So, the bases all react with acids to form salts and all of themproduce hydroxide ions in aqueous solution.

Hydroxide ions are simply part of a water molecule called OH,where it represents a water molecule which hashad one hydrogen atom stripped off of it.So this hydroxide ion in aqueous solution is what characterizes the base.Strong bases like strong acids are usually ionic compounds whichcompletely yield their hydroxide ions to the solution.In other words, the solution reaction completes to completion.So nearly all of the hydrogen, the hydroxide is released as ions.Examples of this, I already gave, are sodium hydroxide.Weak bases are covalent compounds, also known asweak electrolytes, which accept hydrogen ions from the water molecules.Examples of this are things like ammonium hydroxide.Here is another incomplete reaction and some ammoniaremains simply dissolved in the water.And this is a fairly complicated reaction.What happens here is that the ammonia ion stays togetheras an NH3 and the hydroxide forms as an OH.I don't want to draw this out for you, but if you take NH3plus H2O, you'll find that it has exactly the same numberof atoms in it as NH4 plus OH, and you might want to explore that reaction.

The neutralization reactions as you might expect are thosewhere an acid and a base neutralize each other.The reaction is that acid plus a base equals a water plus salt.One example of this would be hydrochloric acid, HCl,plus sodium hydroxide, NaOH, which combineto form water, H2O, and salt NaCl.And again, you might want to try writing thisreaction down or looking it up in the textbook.But, what's going on here is that the essential reactionbetween the acid and the base is the combination of the hydrogenion of the acid with the hydroxide ion of the base to form a molecule of water.In other words, H plus OH simply becomes H2O.The other ions remain in solution in salt water.And so, you get a nonmetal from the acid, and a metalfrom a base, gives you a salt like sodium chloride.Sodium is the metal, chlorine is the nonmetal.

Well, that's it for Program 29.We've covered a lot of material in this program, and I know you'regoing to have to refer to your textbook to see some of these chemical reactions.You're not responsible for knowing all the chemical reactions.What's important here is understanding, first of all,how the periodic table explains all of these different things,including chemical bonding, the electron patterns, the typesof chemical bonds, and electrolytes, acids and bases.So, that's more important than anything else.The next thing to remember is that what we've seen here is themarriage of physics and chemistry, and this marriageis a significant one because it's what puts together allof the physical sciences under one umbrella.So, I don't know what else to say about that, except, remember,when it comes to science, get physical.Bye.So, you've been very quiet since the beginning of the program.Hello, are you sleeping, hello, anybody in there?Music