The nucleus of an atom contains most of the mass of the atom. The nucleus is composed of N (the neutron number) neutrons and Z (the atomic number) protons. Together neutrons and protons are referred to as nucleons. There are Z + N = A (the atomic mass number) nucleons. A nucleus is usually designated by the symbol AX where X is the standard chemical symbol for the element of atomic number Z. For example the nucleus with Z = 6, N = 6, and A = 12 is represented by the symbol 12C.

Isotopes are nuclei with the same Z but different N and A. Examples of isotopes are 12C, 13C , and 14C.

Isobars are nuclei with the same A but different N and Z. Examples of isobars are 12C, 12N , and 12B.

Isotones are nuclei with the same N but different A and Z. Examples of isotones are 12C, 13N , and 14O.

Nuclei are bound systems of nucleons. The mass of the nucleus is less that the sum of the masses of the constituent nucleons. The binding energy is given by

BE = (Nmn + Zmp - mN)c2

where mN = mass of nucleus, mn is the mass of the neutron, and mp is the mass of the proton. This is a direct consequence of Einstein's famous formula E=mc2.

Nuclei that are stable and naturally occurring have radii given by

R = RoA1/3 , Ro = 1.2 $\cdot$ 10-15 m = 1.2 fm.

The unit fm is the femtometer but is often called the "fermi".

MultiMedia Physics 2000