composition and structure Atoms composition and structure

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

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

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

Insights from Chemistry Existence of elements Law of definite proportions

Cathode Rays Source: Griffith, Physics of Everyday Phenomena

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

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

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

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

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

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

Rutherford’s Experiment most Source: Griffith

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

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

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

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)

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

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

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.

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