1. Monday(05/03): Intro. to Ch.28 - The Atom. Review of Models

of the Atom. The Bohr Model. HW: Read and Study pages 747-52, then

solve problems 43, 44, 45, and 46 on page 771.

2. Tuesday(05/04): Quantized Energy. HW: Read and Study pages

753-56, then solve problems 47, 48, and 51 on page 771.

3. Wednesday(05/05): The Planetary-Quantum Atomic Model (Applications

and Problem-Solving). HW: Study page 769 and then solve problems 52,

57, 62, 63, and 65 on pages 771-2.

4. Thursday(05/06): LAB on Atomic Model Simulation. HW: Process lab

data. Report due on Tuesday.

5. Friday(05/07):  Post-lab Discussion. HW: Write Lab Abstract.

Report due on Tuesday.

6. Monday(05/10): REVIEW Ch. 28 - The Atom. HW: Complete

Review Handout.

7. Tuesday(05/11): TEST on Ch.28 - Quantum Theory. HW: Visit the

web-site for notes and plans for Ch.29 - Solid-State Electronics.

1. So far we are familiar with three atomic Models. The first was that of

Democritus (460-370) BC, the ancient Greek philosopher, who proposed

that the smallest unit  of matter was a small, single, indivisible particle,

"atomos."

2. This belief lasted for almost 2300 years, until the 1890's when the

British Physicist, J. J. Thomson discovered the electron using his CRT.

His model was known as the "Plum Pudding" Model.

3. In the early 1900's, Ernest Rutherford, also from England proposed his

Planetary Model, based on his "Gold Foil" Experiment. This atomic model

was characterized as mostly empty space, but with a very dense, positively

charged nucleus, and orbiting electrons.

4. While the Rutherford (Planetary) model focused on describing the nucleus,

the electron was depicted as an orbiting planet. The flaw with the planet-like

model is that an electron particle moving in a circular path would be

accelerating. 

5. An accelerating electron creates a changing magnetic field. This changing

magnetic field would carry energy away from the electron, eventually slowing

it down and allowing it to be "captured" by the nucleus.

6. In 1913, the Danish physicist Niels Bohr (1885-1962) managed to

explain the a new atomic model as an extension of Rutherford's

description of the atom.

7. Bohr agreed that the negatively charged electrons revolve about the

positively charged atomic nucleus because of the attractive electrostatic

force according to Coulomb's law.

8. But the electron can be taken not only as a particle, but also as a

de Broglie wave (wave of matter) which interferes with itself.

9. The orbit is only stable, if it meets the condition for a standing wave:

The circumference must be an integer multiple of the wavelength.

The consequence is that only special values of radius and energy are allowed.

10. According to classical electrodynamics, a charge, which is subject to

centripetal acceleration on a circular orbit, should continuously radiate

electromagnetic waves.

11. Thus, because of the loss of energy, the electron should spiral into the

nucleus very soon. By contrast, an electron in Bohr's model emits no energy,

as long as its energy has one of the above-mentioned values.

12. However, an electron which is not in the lowest energy level (n = 1), can

make a spontaneous change to a lower state and thereby emit the energy

difference in the form of a photon (particle of light).

13. By calculating the wavelengths of the corresponding electromagnetic

waves, one will get the same results as by measuring the lines of the

hydrogen spectrum.

14. We must not take the idea of electrons, orbiting around the atomic

nucleus, for reality. Bohr's model of the hydrogen atom was only an

intermediate step on the way to a precise theory of the atomic structure,

which was made possible by quantum mechanics and quantum

electrodynamics.

15. Still, the most important properties of atomic and molecular structure

may be exemplified using a simplified picture of an atom that is called the

Planetary Quantum or Bohr Model.

16. Again, this model was proposed by Niels Bohr in 1915 and although it

is not completely correct, but it has many features that are approximately

correct and it is sufficient for much of our discussion.

17. The correct theory of the atom is called Quantum Mechanics; the Bohr

Model is an approximation to quantum mechanics that has the virtue of

being much simpler to understand.

18. In the Bohr Model the neutrons and protons occupy a dense central

region called the nucleus, and the electrons orbit the nucleus much like

planets orbiting the Sun (but the orbits are not confined to a plane as is

approximately true in the Solar System).

19. This similarity between a Planetary Model and the Bohr Model of the

atom ultimately arises because the attractive gravitational force in a solar

system and the attractive Coulomb (electrical) force between the positively

charged nucleus and the negatively charged electrons in an atom are

mathematically of the same form.

20. The form is the same, but the intrinsic strength of the Coulomb interaction

is much larger than that of the gravitational interaction; in addition, there are

positive and negative electrical charges so the Coulomb interaction can be

either attractive or repulsive, but gravitation is always attractive in our present

Universe.

21. Based on the spectrum of atomic hydrogen and the fact that each gas has

a unique emission and absorption spectrum, Niels Bohr proposed his Quantum-

Mechanical Atomic Model instead of Rutherford's Planetary Model.

22. He proposed that electrons can move from one energy level to another by

absorbing or emitting photons. His equations were: (a) for orbital radii.

r_n = 5.3x10^-11 m x n² , and (b) for the ionization energy associated with each

level,E_n = -13.6 eV x 1/n² , with n = 1,2,3,... , the energy level number.

The electron-volt (eV) is the energy unit for electrons, 1 eV = 1.6x10^-19 J .

23. Werner Heisenberg (1901-1976) determined that it is not possible to know

the exact position and momentum of the electron, the Uncertainty Principle.

24. Arthur Holly Compton (1892-1962) bombarded a graphite block with X-rays

demonstrating the momentum of photons (The Compton Effect ). The equation

is mv = p = h/λ .

25. James Chadwick (1891-1974) an original member of Rutherford's research

team proved the existence of neutrons in 1932.

26. Light Amplification by Stimulated Emission of Radiation (LASER), which

was explained by Einstein in 1917, was invented in 1960. Laser light is very

directional, powerful, monochromatic, and coherent, making it very useful.

27. And to get full credit for your homework make sure you are following

these steps

(i) read the problem and identify the given variables

(ii) determine what you are asked to solve for

(iii) find the correct formula to use

(iv) use algebra to isolate the unknown

(v) substitute-in the given information and simplify.

Answers to Homework: (Scrambled Format.)

THE HONORS PHYSICS ARCHIVES Ch.1: Physics Intro. Ch.2&3: Linear Motion. Ch.4&5: Forces. Ch.6: 2-Dim Motion. Ch.7: Gravitation. Ch.8: Rotary Motion. Ch.9: Momentum. Ch.10&11: Work&Energy. Ch.12: Thermal Energy. Ch.13: States of Matter. Semester Review. Ch.14&15: Waves&Sound. Ch.16: Study of Light. Ch.17&18: Mirrors & Lenses. Ch.19: Light Interference. Ch.20&21: Electrostatics. Ch.22&23: DC Circuits. Ch.24-26: Electromagnetics. Ch.27: Quantum Theory.