Download 9April2012 Notes

4/9/2012
We began Atomic & Nuclear Physics with C38, pp 367-374. One should know (1) the components of an atom
(protons & neutrons in the nucleus with electrons orbiting in the electron clouds), (2) neutrons and protons are
each composed of quarks, 3 for protons and a different 3 for neutrons, (3) protons and neutrons are held
together at very short distances by a strong nuclear force that is 100 x stronger than the weaker electromagnetic
force that tends to push protons away from other protons (4) that the nucleus’ size relative to the whole atom is
equivalent to a marble on the 50-yard line compared to the entire football stadium (5) electrons are ~ 1/1840th
the size of protons and neutrons (6) protons & neutrons are each 1 atomic mass unit (abbreviated “amu”) (7)
the atomic number on the periodic table is equal to the # of protons for that element (8) the mass listed on the
periodic table is the weighted average mass, thought of in terms of either atomic mass units, equal to the sum of
its protons & neutrons, or as the # of grams of 1 mole of that particular atom. (9) The reason the mass is not a
whole number is because it is a weighted average of all the isotopes of that given element. An isotope is an
atom with a different # of neutrons than the most common form, and therefore, has a different mass. (10) To
calculate an atom’s # of neutrons then is easy: Round the atomic mass to a whole # and then subtract the
atomic # from it. (11) A mole is a unit of quantity. It is equal to Avogadro’s Number which is 6.022 x 1023 atoms
or molecules or particles. Because each element has a different # of protons & neutrons, each will have a
different molar mass (another name for the “weighted average mass”). For instance, one mole of hydrogen
molecules, H2, is 2 grams. On the other hand, 1 mole of gold, Au, is 197 grams. (12) Metals are held together by
metallic bonding. This is unique in that metal cations are surrounded by a sea of mobile electrons. (13) The
ability of these electrons to be excited to a higher energy state when hit by light is called the “photoelectric
effect”. Not all light, however, can excite electrons to give off photons of energy when excited. {Einstein coined
the term “photon”.} (14) Red light has too low a frequency to do so, which is why poachers use red lights to
prevent their gun barrels from shining in the dark. It is also why you need red filters on lights when walking
beaches where sea turtles nest. Regular lights confuse the newly hatched sea turtles because they are
programmed to follow the moonlight back into the sea; street/house/car/flash lights would attract them to
dangerous roads/traffic. (15) The photoelectric effect forced scientists to realize light was not just a wave but
that it also had particle properties. The dual nature of light is mirrored by the dual nature of all matter. (16) de
Broglie proposed that all matter has wavelike properties.
This sets the stage for Quantum Physics.