The nucleus is composed of Z protons and N neutrons. The atomic number Z determines the chemical properties of the atom. The mass number A=Z+N determines approximately the mass of the nucleus in atomic mass units.
- A particular nucleus defined by A and Z is known as a nuclide.
- Elements are defined by the atomic number Z.
- Atoms (or nuclei) with the same Z but different N are known as isotopes. These have different masses but the same chemical properties.
- Elements with the same A but differing Z (and N) are known as isobars. These will have approximately the same mass but different chemical properties (determined by Z).
- Nuclei with the same N but differing Z are known as isotones.
- Nuclides are symbolically denoted by
(or more fully, 
), where El is the usual chemical symbol for the element.
It does not follow that nuclei can be found for every given N and Z. Indeed, we find at most two stable nuclei for a given A. (The word stable has several meanings when applied to nuclei. Here we mean beta-stable, referring to relative stability amongst a set of isobars). Nuclei that have been observed in nature and experiments fall in a relatively narrow band in the (N,Z) plane around the stable nuclei. Obviously we would like to understand this distribution.
Information provided by: http://www.phy.uct.ac.za