1. composition and structure
Atoms
composition and structure

2. Homework Problems Now so I don’t forget to tell you
Due next Friday (why not)?
3.1, 3.18, 3.40, 3.41, 3.42, 3.47

3. Atomic Hypothesis
“If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on the the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is…
“all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.”
R.P. Feynman, The Feynman Lectures on Physics, vol.1

4. Atom Greek root atomos a (not) + temnein (to cut)
Not actually indivisible, but still a fundamental arrangement of matter!

5. Insights from Chemistry
Existence of elements
Law of definite proportions

6. Cathode Rays
Source: Griffith, Physics of Everyday Phenomena

7. J.J. Thompson’s Experiment
Source: Griffith, Physics of Everyday Phenomena, 4 ed.
charge
mass
Measured
Ratio of cathode rays

8. Forces on the cathode rays
The force from the electric field is
A. Upward B. Downward
C. Toward you D. Away from you

9. Forces on the cathode rays
The force from the magnetic field is
A. Upward B. Downward
C. Toward you C. Away from you

10. Forces on the cathode rays
Which field exerts the stronger force on the beam?
A. electric B. magnetic

11. Millikan’s Experiment+ – E
oil drops
Charge of drop determined by mass and E
Always a multiple of 1.6  10–19 C

12. So... Matter contains electrons They are small and negatively charged
mc2 = 511 keV
charge –1.60  10–19 C
There must be positive stuff in there too

13. Rutherford’s Experiment
most
Source: Griffith

14. So...
A lot of the mass of matter must be in small, dense, heavy pieces with + charge
A lot of empty space too

15. And... There are neutral nucleons too We now call them neutrons
Discovery described in chapter 17

16. Nucleons mc2 = 938.3 MeV mc2 = 939.6 MeV
Source: Griffith, Physics of Everyday Phenomena

17. Identifying Nuclei Number of protons = atomic number Z
Number of neutrons N
N + Z = mass number A
Isotope name: Name-A (e.g. Carbon-12) or AX (e.g. 12C)

18. Atomic Structure Protons and neutrons in a small, dense nucleus
Electrons in “orbitals” around the nuclei

19. Preview
Protons don’t eject each other because of a strong nuclear force
Electrons don’t collapse to the nucleus because of quantum uncertainty
Electrons have different energies because of the Pauli exclusion principle

20. Atomic Mass Unit About the mass of a nucleon in an atom
Strictly, 1/12 the mass of a 12C atom
Why 12C/12 instead of p? Mass deficit.

21. Avogadro’s Number NA Number of 12C atoms in 12 g 12C NA = 6.022  1023
NA of something  1 mole