Sci 122 Program 28 Empty Atoms

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Empty Atoms
Program 28
Lesson 4.6

Text References

Spielberg & Anderson 260-298

Booth & Bloom 319-330; 374-409

Coming Up



1. Introduction

2. Pieces of Atoms

3. The Fruitcake Atom

4. The Nuclear Atom

5. Planck's Quantum Hypothesis

6. Photoelectric Effect

7. The Bohr Atom

8. Matter Waves

9. Paradigm Clash

10. Summary & Conclusions

Before we're done with this program we will have studied the composition of atoms and the early attempts at understanding their structure. We will see how atoms can be studied by shooting atomic bullets at them and how they behave quite different from what we expect. The photoelectric effect will show us that their behavior is consistent and will lead us to the understanding of the atom as a quantitized system. The relationship between waves and particles will be extended from light to matter in general as we engage in a paradigm clash, the resolution of which allowed a deeper understanding of atoms and their properties, and the role of electrons in chemical bonding and the macroscopic properties of matter.


1. How are cathode rays produced, where do they come from, and what are their properties?
2. What significance did the discovery of the electron have for atomic theory?
3. What was the Fruit Cake model of the atom?
4. Name three types of radioactivity and list the characteristics of each.
5. What problems did the discovery of radioactivity cause for physical scientists?
6. How is the radioactivity of a substance such as uranium affected by chemical combination and heat?
7. What does it mean to say that Rutherford was the first of the "big" scientists?
8. On what evidence did Rutherford base his nuclear model for the atom?
9. In what way is the nuclear model of the atom inconsistent with electromagnetic theory?
10. Briefly describe the Photoelectric Effect and the problems it raises for the wave theory of light.
11. What is meant by a "quantum" of energy?
12. What did Millikan's oil drop experiment do?
13. How did Bohr solve the problem of the nuclear atom?
14. How is the Bohr atom different from Rutherford's nuclear atom?
15. Describe the relationship between the Bohr atom and the hydrogen spectrum.
16. Discuss the statement, "The atom is mostly empty space."


1. Describe the pieces of atoms discovered late in the nineteenth century and their properties.
2. Compare and contrast the fruit cake, nuclear, and Bohr models of the atom
3. Describe the salient features of Planck's quantum hypothesis
4. Describe the problems associated with photoelectric effect and Einstein's quantum solution
5. Describe the relationship between waves and matter at the quantum level
6. Describe the paradoxes and controversy surrounding the wave-particle duality

1. Introduction

This material represents a radical departure from the physics we have come to know and love. It is included anyway because historically it is important in our modern understanding of everything from chemical bonding to lasers.
We will not have to go through all of the paradigm shattering trauma which accompanied the acceptance of these ideas within the scientific community. There are still many who do not accept this as the final truth about the very small world inside the atom.
The structure of the atom was investigated using radioactivity and ultimately led to the discovery of new kinds of matter with new properties. These small scale properties led to understanding of atomic spectra and chemical properties of elements and would lead to the quantum revolution in physics which became the new paradigm of subatomic particles and forces.

2. Pieces of Atoms

2.1. Cathode Rays

2.1.1. Heinrich Geissler, glassblower (1814-1879) invented high vacuum pump, evacuated a glass tube
2.1.2. Plucker (1801-1868) sealed wires into the tube, connected to a battery discovered that electricity flowed through the tube forerunner of fluorescent lights, TV tubes, neon lights

2.1.3. Crookes (1832-1919) used induction coil to produce cathode rays travel in straight lines, cast shadows (see Figure 11-3, p.239) cathode rays produced by many different material have similar properties

2.1.4. provoked many questions and much further study waves or particles? carry negative charge (Perrin 1895) where do they come from? supply of electrical energy produced unlimited cathode rays how could different substances produce the same kind of rays?

2.2. The Electron

2.2.1. William Thomson (1856-1940) cathode rays deflected by magnetic and electric fields confirmed negative charge used electric and magnetic fields together to show that cathode rays are streams of particles measured ratio of charge to mass (e/m) rays from all types of material have same e/m 1.76x1011 coulombs/kilogram suggested that cathode rays are streams of particles broken off from the atom atoms are not the ultimate building block of matter called the particles electrons, building blocks of atoms

2.2.2. Robert Millikan (1868-1953) used oil drop experiment to measure charge of electrons series of experiments between 1909 and 1916 balanced electrical and gravitational force on small charged oil drops

Millikan's Oil Drop Experiment.

The charge of the electron is measured by balancing the gravitational force of statically charged oil drops with the coulomb electrical force.

From this data the total charge on the drops can be calculated. The charge on the electron can be determined mathematically. The smallest number which is a factor of all of the individual charges is the charge on the electron. It is 1.6 x 10-19 coulombs. found that all drops carried multiples of a certain charge charge comes in packages of a minimum size e = 1.6E-19 coulombs sugar lumps in coffee analogy combined with Thomson's e/m measurements to calculate mass of electron mass of electron = 9x10-31 kg (very small) about 1/1840 the mass of a hydrogen atom Millikan won Noble Prize

2.3. X-rays

2.3.1. Roentgen (1845-1923) discovered by accident
2.3.2. studying cathode rays in a Crookes tube covered with black paper
2.3.3. noticed glow on fluorescent material on bench when lights turned out
2.3.4. concluded it came from Crookes tube cathode rays do not travel outside tube for long distances no source of UV anywhere in lab called it X-rays (unknown rays)

2.3.5. published results led to immediate medical applications
2.3.6. established source and nature of rays originate at end of Crookes tube where cathode rays strike not deflected by magnetic or electric field, so not charged particles travel in straight lines, darken a photographic plate, have great penetrating power wavelength is considerably shorter than UV light, frequency is high can be polarized, diffracted, reflected, refracted like waves

2.4. Radioactivity

2.4.1. Becquerel (1852-1909) was curious about fluorescence caused by X-rays
2.4.2. certain natural crystal fluoresce (glow), especially when illuminated by UV light
2.4.3. also knew that uranium ores darken photographic film when exposed to light also have penetrating power, like X-rays

2.4.4. discovered by accident that light is not necessary wrapped uranium salts in black paper, stored in drawer affected film even in the dark

2.4.5. discovered other properties too uranium salts continue to emit rays for indefinite period of time radiate whether in crystalline form, in compounds, or in solutions radiate in proportion to uranium content, pure uranium even more so

2.4.6. significance overshadowed by X-ray discovery
2.4.7. Marie and Pierre Curie did further studies Thorium displayed radiation similar to uranium uranium ore more radioactive than suggested by uranium content isolated two new chemical elements (polonium and radium) polonium 400 times more radioactive than uranium, radium a million times showed that radioactivity is spontaneous in certain elements unaffected by heat, pressure, chemical combination opened question of source of energy: is energy conservation violated?

2.4.8. Ernest Rutherford (1871-1937) showed that radioactivity had two components used electric and magnetic fields called them alpha and beta alpha particles are heavy, carry positive charge, stopped by thick paper, resemble helium atoms beta particles are light, carry negative charge, stopped by thin piece of metal, resemble cathode rays third component (gamma rays) discovered later, have no charge, resemble high energy X-rays

3. The Fruitcake Atom

3.1. apparent that the atoms contains both positive and negative pieces

3.1.1. electrons can be knocked off in cathode ray tube
3.1.2. pieces of atoms ejected in radioactive decay

3.2. how are positive and negative charges arranged?

3.2.1. chemical and thermodynamic models of atom did not address structure

3.2.2. simplest structure is positive atom with negative electrons embedded like fruit in fruitcake

The Fruitcake Atom

The earliest model of the atom pictured it as a solid sphere of positive charge with negatively charged electrons embedded in in like raisins in a cookie.

These electrons were thought to be the source of the cathode rays, knocked out of the atom by high voltage electricity. electrons kicked out of atom by electrical energy does not explain atomic spectra nor chemical properties

4. The Nuclear Atom

Ernest Rutherford and Big Science

Ernest Rutherford was the first of the "big" scientists. Not that he was overweight, but rather that his experiments took on large proportions. His laboratory employed many professional scientists who designed and carried out experiments under Rutherford's direction.

This was a departure from the traditional small laboratory with one man designing, building, and conducting the experiment single-handedly or with laboratory assistants.

Rutherford's research not only revealed the structure of the atom, it also became the paradigm for research science worldwide.

4.1. Rutherford used alpha bullets to shoot at gold atoms

4.1.1. gold is most dense substance known, so atoms should be massive and close together

4.2. expected to observe scattering as alphas collide with atom
4.3. instead found that most alphas passed through without deflection

4.3.1. occasionally one was scattered, only rarely reflected straight backwards

4.4. calculated size of nucleus of atom to be 1/1000 the size of the atom itself

4.4.1. from ratio of deflected vs. undeflected atoms
4.4.2. the atom is mostly empty space positively charged nucleus contains all positive charge and most of mass negative charges in orbit around positive nucleus

4.5. model is inconsistent with electromagnetic theory

4.5.1. electron in circular orbit is accelerated should radiate energy and spiral into nucleus short lifetime calculated (about 1 nanosecond)

4.5.2. atoms do not decay this fast, in fact most are stable forever

5. Planck's Quantum Hypothesis

5.1. from study of blackbody radiation in heated box

5.1.1. energy exchanged between radiation and walls of box in equilibrium
5.1.2. contradiction between two different theories and observation
5.1.3. wavelength of maximum intensity depends only on temperature shorter wavelength implies higher temperature

5.2. ultraviolet catastrophe

5.2.1. theory predicted that radiation would shift towards shorter wavelength (higher frequency)
5.2.2. eventually would become invisible
5.2.3. violates conservation of energy

5.3. energy quanta

5.3.1. Planck saved conservation of energy with simple assumption
5.3.2. intended as mathematical tool to mesh theory with observation
5.3.3. energy can only be exchanged in proportion to the frequency of the radiation E = hf (h = Planck's constant, 6.63E-34 J s) quantum = smallest amount of energy that can be exchanged ascribes particle like properties to radiation higher frequency means larger exchange quantum fewer high energy states available, so little energy in high frequencies

5.3.4. observed spectrum is balance between large number of low energy quanta, each with small energy small number of high energy quanta, each with large energy

6. Photoelectric Effect

6.1. it was observed that light can eject electrons from the surface of a metal under certain conditions
6.2. energy of electrons depends on frequency of incident light and type of metal

6.2.1. below a certain frequency no electrons are ejected (cutoff frequency)
6.2.2. above the cutoff frequency electrons have limited maximum energy
6.2.3. increasing intensity (amplitude) of light increases number of ejected electrons but does not affect their maximum energy in classical wave theory energy of waves is proportional to amplitude how can electrons absorb just the right amount of energy regardless of intensity? how can energy be stored in wave until the right amount of energy has been absorbed?

6.3. Einstein gave simple answer

6.3.1. related to Planck's quantum. energy of light is proportional to frequency each material has a given energy barrier to overcome (work function) light with less energy than work function cannot eject electrons light with more energy than work function ejects electrons Emax = hf - work function

6.3.2. light exchanges energy only in bundles proportional to its frequency
6.3.3. generalized Planck's quantum to all types of EM radiation light has wave properties in transmission, particle properties in energy exchange

7. The Bohr Atom

7.1.Neils Bohr solved problem of the nuclear atom

7.1.1. assumed that there must be a new principle at work, since we observe that atoms are mostly stable

7.1.2. supposed that the momentum of the electron in the atom is quantitized along with its energy orbits are stable only for certain values of momentum used Planck's constant as quantum of momentum

7.1.3. did simple calculations to determine the radius of the hydrogen atom results agreed with measured size of hydrogen atom

7.1.4. did simple calculation to determine the energies of the allowed quantum states showed that difference in energy levels corresponds to wavelengths of hydrogen spectrum derived Rydberg formula

7.2. atoms absorb and emit radiation which corresponds to differences in energy levels of quantitized orbits

7.2.1. absorption corresponds to removing light of a specific energy (wavelength) from incident spectrum
7.2.2. emission corresponds to electron losing energy to fall to a lower quantum state

8. Matter Waves

8.1. Louis DeBroglie asked a simple but creative question

8.1.1. part of Ph.D. thesis, awarded Nobel Prize

8.2. if light waves can have particle-like properties, can particles (electrons) have wavelike properties

8.2.1. calculated "wavelength" of beam of electrons

8.2.2. found that electron in atomic orbit is in resonance only orbits for which the circumference is a multiple of the wavelength are allowed consistent with Bohr's atom

8.3. within a few months beams of electrons were shown to have diffraction patterns

8.3.1. calculated wavelength matched DeBroglie's predictions
8.3.2. basis for electron microscopes beams of electrons are focused through magnetic lenses to form magnified images similar to light microscope requires TV screen to see images electrons have smaller wavelength than light for a given energy smaller wavelength allows smaller objects to be seen

9. Paradigm Clash

9.1. much controversy followed concerning the reconciliation of the wave/particle duality
9.2. quantum mechanics is the science of the very small
9.3. led to deeper understanding of chemical properties and structure of the atom

10. Summary & Conclusions

In this lesson we have learned how the structure of the atom was discovered. First there were pieces of atoms; the electron and radioactive particles which possessed electric charges and distinct charge to mass ratios.

Planck's quantum and the photoelectric effect indicate that radiant energy in the form of light is quantitized into packets or bundles, the size of which depends on the frequency or color of the radiation.

Bohr's suggestion that the momentum of electrons in the atom is also quantitized led him to a model of the atom which explains the Balmer spectrum using the concept of the quantum of radiant energy.

The explanation for the quantum nature of electrons in the atom came with De Broglie's discovery of the wave nature of matter which. This explained the quantitized momentum of Bohr's atom in general terms. This wave nature of matter was confirmed when a beam of electrons was observed to display diffraction similar to light waves.

This duality of waves and particles created a serious problem for physics as a science. Many years of speculation, debate and research, both experimental and theoretical followed in a very exciting period, the most productive in the history of science.