Nucleus
Ernest The Rutherfords proposal that the atom consisted of negatively charged electrons
orbiting a positively charged nucleus triggered a great deal of curiosity about the
structure of that nucleus. It is important to understand how small it really is. If a hydrogen
atom were the size of San Diego, the nucleus at the center would be smaller than a
baseball. The atom as we understand it consists almost entirely of empty space.
The phenomenon of radiation emitted from certain rare minerals had been studied long
enough to conclude that it came chiefly in three particle-like forms named alpha ( ),
beta ( ), and gamma ( ). Alpha particles were heavy, comparatively slow, and positively
charged, judging by they way they deflected in a magnetic field. They could be blocked
effectively by a sheet of paper. Beta particles were much faster and lighter, negatively
charged, and required something denser, like wood or skin, to stop them. Gamma
particles appeared to move at the speed of light. They were uncharged and could
penetrate several inches of lead. Clearly, these particles were coming out of the nucleus,
so the nucleus must be composed of them, except the beta particle turned out to be
nothing more than a very fast-moving electron and the gamma particle was found to be a
high-frequency photonjust another kind of light.
Accelerating alpha particles and smashing them into atoms resulted in the discover of a
new particle that had exactly the same charge as an electron, but it was positive instead of
negative and had a mass roughly 2000 times greater. Stranger still, it seemed to be
identical to the nucleus of the hydrogen atom with the electron stripped off. It was called
the proton. Another particle was found later having a nearly identical mass as the proton,
but a neutral charge. It was named the neutron. The alpha particle was determined to
consist of a bundle of two protons with two neutrons, and was identical to the nucleus of
a helium atom. It was subsequently determined that all atoms are built out of these two
nucleons, and so the persistent picture of a conglomeration of spherical nucleons at the
center of the atom was born.
The number of protons determines the number of electrons through electrostatic
attraction. Since the electrons determine the chemical nature of the atom, the number of
protons was labeled the atomic number and inexplicably given the symbol Z. Since
neutrons have essentially the same mass as protons, and since electrons are vastly lighter
than either, the mass of the atom is determined by the sum of the nucleons. This is called
the mass number and given the symbol A. The approximate radius of the nucleus in
femtometers (10 -15 m) can be found using r = 1.2 A1/3. Different numbers of neutrons do
not affect the chemical identity of the element. These different varieties of the same
element are called isotopes. For the one-hundred-plus elements found so far, more than a
thousand isotopes are known. The vast majority of these are unstable (radioactive), so
there is clearly a magic relationship between the number of protons and neutrons to make
an isotope stable. Above element 82 (lead), all known isotopes are radioactive.
Additionally, elements 43 and 61 (technetium and promethium) do not have stable forms.
As a rough approximation, you can estimate the atomic weight (mass) of an element from
its atomic number using Z(2+Z/150) and rounding downward to the preceding integer.