The nature of the atomic nucleus was first elucidated experimentally by Ernest Rutherford in 1911. He observed the scattering of -particles (helium nuclei) from a thin gold foil (see schematic below):
Rutherford estimated the ``diameter'' of an atom to be approximately 10 cm and that of the nucleus to be approximately 10 cm. He proposed a model of the atom as consisting of a small, dense nucleus surrounded by enough electrons to yield an overall charge-neutral aggregate. That is, if the nuclear charge is , then the atom must possess electrons. The model later introduced by Bohr, and even the current quantum mechanical picture of the atom (which you will see next semester), were built upon Rutherford's original model.
As mentioned in the first lecture, we know today that the atomic nucleus
is composed of protons and neutrons. If a nucleus has protons
and neutrons, it is said to have a mass number equal to .
Thus, if represents the chemical symbol of some element, then
its nucleus is represented by