Lecture notes 28 June
Atomic
physics:
Early
concepts of the atom
Dual
nature of light
Bohr
theory of the hydrogen atom
MicroWAVE
ovens, x-rays and lasers
Heisenberg’s
uncertainty principle
Matter
waves
Electron
cloud model of atom
Now, how
to present it?
Chapter
9: A coupla things you all need to know.
1) Beginnings of quantum theory: Light, which was once
thought to be purely a wave, also behaves like a particle. In particular in its interaction with matter,
light is absorbed and emitted as if it were a particle. Max Planck discovered the first quantum effect in 1900 when he noticed that if he added up the energies at different frequencies being emitted from a heated ideal radiator he could get the correct spectrum (the spectrum observed in experiments) only by using a discrete sum rather than a continuous sum (summation symbol Sigma used instead of contiunuous sum of calculus integration process, see?). Einstein in 1905
extended the discovery by describing the photoelectric effect in terms
of light behaving as a particle. The
particle of light soon came to be called the photon (coined by G. N. Lewis in 1916). This was the beginning of quantum mechanics,
which also says that particles like protons, neutrons and electrons also
sometime behave like waves (de Broglie and Schrodinger). When you try to find the precise position and
speed of one of these particles, you find that it behaves like a wave—it
doesn’t have a precise position and speed.
This is the basis of the uncertainty principle, which is also called the
indeterminacy principle, which says, for instance, that the position
and speed of a particle can’t be determined simultaneously with unlimited accuracy.
2) The Bohr model of the
hydrogen atom. The Rutherford model of
the atom (1911) claims that electrons orbit the nucleus somewhat like planets
orbit the sun, and came about from experiments involving shooting alpha
particles at gold foil. Why gold? Didn’t oxidize, but remained “pure” so it
would give the least complicated experimental results. Even with the Rutherford nuclear model, the neat, discrete
spectrum emitted by atoms was not explained. Light with precise frequencies is
observed to be emitted from atoms, not a smeared out spectrum, like light
from a rainbow or sunlight refracted through a prism. Another problem
was if electrons orbit like planets, they have centripetal acceleration. That should cause them to lose energy and
fall into the nucleus. But atoms are
stable! In 1913, Niels Bohr proposed that electrons in atoms
occupy stable orbits called energy levels, and that electrons only radiate when
they fall from a higher orbit to a lower one. When an atom absorbs light, an
electron jumps from a lower energy level to a higher one. (Draw picture.) Also, for the rotational and vibrational
states of molecules, longer wavelength radiation is absorbed and emitted. Microwaves are a good example. Also for heavy atoms, x-rays can be emitted
by an electron being knocked out of lowest energy level and one from a high
energy level falling into that lowest level.
This spontaneous jumping of electrons to a lower energy level is not suprisingly called "spontaneous emission." Then there are lasers, L A S E R, light amplification by stimulated emission of radiation. A photon hits an atom in an excited state, knocking the excited electron down to a lower state, and in the process stimulating the emission of a photon which goes on off with the photon that came in. Stimulated emission, but not the laser itself, was predicted in 1917 by Einstein in his superdooper derivation, via thermodynamic equilibrium of light and atoms in a box, of the Planck spectrum.
Chapter
10: This is Nuclear physics, not rocket
science. Rocket science has big
equations, and is complicated. Nuclear
physics has little equations and is really pretty simple.
Hydrogen
is the simplest atom. One electron orbiting one proton. If there were just the gravitational and the
electromagnetic forces, hydrogen would be the only element in existence. But there is a force stronger than the electromagnetic
force that causes protons and neutrons to clump together, and form atoms
heavier than hydrogen. This is the third fundamental force we’ve talked about,
the first two being gravity and electromagnetism. This force that holds nuclei
together is called the strong force, because it’s stronger than
electromagnetism.
But
the strong force is only operative over a short range, and the electromagnetic
force operates over an infinite range. (So does gravity, but it’s too weak to
have an influence at the atomic level.)
The bigger a nucleus gets, the less effective the strong force is in
holding it together, because the electrical repulsion of the many protons
overwhelms the short-range strong force.
For atoms with more than 83 protons, the electrical repulsion between
protons starts to overcome the strong force, and the nucleus decays
radioactively. So atoms with atomic
numbers greater than 83 are unstable.
They decay into more stable atoms with fewer protons.
What
is the atomic number? The number of
protons in an atom. It determines the
place of an element in the periodic table.
The atomic mass is the number of protons plus the number of neutrons in
an atom.
Radioactivity
and half-life.
The
last and least-strong force that is known today is called the weak force,
so called because it’s weaker than the electromagnetic force. It causes a nucleus to emit electrons,
turning a neutron into proton in the process. Also recall that a free
neutron—one that’s outside a nucleus—decays with an average lifetime of about
11 minutes, so it also decays by emitting a fast-moving electron, which is
called a beta ray. This type of radioactivity is important because it involves
all three fundamental particles that make up an atom—proton, neutron and
electron. There are other types of
nuclear radiation also. In Greek
alphabetical order the three common types are:
alpha rays (helium nuclei, 2 protons and 2 neutrons) beta rays
(electrons), and gamma rays (very high energy electromagnetic waves). There is
one more type, not as common but very important, and that is neutrons. X-rays also are dangerous (ionizing), but not
microwaves—they’re low energy. X-rays
produced by – draw picture. HV supply,
vacuum tube, electrons attracted to postive plate, x-rays produced when atoms
in plate are hit by electrons. Like TV,
but it aint as high voltage. (wrote on the board some other stuff, just table
10.5 mainly: positrons added to list of nuclear radiation.)
_____________________
stopped here in class______________________________
Half-life. What’s the half-life of a hamburger? "I only want half a hamburger." "Well, I only want half of a half of a hamburger." "I only want half of a half of a half of a hamburger." Which can go on indefinitely. If you actually ate half of a hamburger in one second, handed the remaining half to your brother, who ate half of that in one second, which he handed back to you (now one fourth is left), which you ate in one second... and so on, the half life of the hamburger would be one second. The rate at which the hamburger is eaten, you see, not constant. It slows down. In order for the concept of half-life to be valid, the rate must be proportional to the amount remaining. (Well, you could use a 1/10th life or a 1/3rd life, or similar, but half-life is the standard measure.)
Later: Nucular
reactions and uses of radionuclides. Fission
and Fusion.
Biological
effects of radiation. Alpha, beta, gamma, neutrons
and x-rays are ionizing radiation. Ionizing means they can knock electrons out
of an atom, or even hit the nucleus and change its electrical charge. The result in either case is that the atom
becomes an ion, meaning it isn’t electrically neutral. Ion means “an atom,or chemical combination of
atoms, that has a net electric charge.”
