#### CHAPTER 13 NUCLEUS

Nucleus

Rutherford proposed that the entire positive charge and nearly the entire mass of atom are concentrated in a very small space called the nucleus of an atom.

The nucleus consists of protons and neutrons. They are called nucleons.

Z - atomic number = number of protons

N - neutron number = number of neutrons

A - mass number = Z + N = total number of protons and neutrons

Nuclear species or nuclides are shown by the notation ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$ where X is the chemical symbol of the species. For example, the nucleus of gold is denoted by. It contains 197 nucleons, of which 79 are protons and the rest 118 are neutrons.