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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
Measured
Ratio of cathode rays
mass
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
oil drops
E
+
–
• 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