The Nuclear AtomThe Nuclear Atom

Rutherford’s alpha-scattering experiment provided evidence of a small, charged nucleus. A narrow beam of alpha particles, all of the same kinetic energy from a radioactive source were targeted at a thin piece of gold foil which was only a few atomic layers thick. The alpha particles were scattered by the foil and detected on a zinc sulphide screen mounted in front of a microscope. Each alpha particle hitting the fluorescent screen produced a tiny spec of light. Counting these specs led to two significant observations:

  1. Most of the alpha particles passed straight through the gold foil with very little scattering, so the atom must be mostly empty space
  2. Very few were deflected through extreme angles so there must be a very small, dense nucleus of positive charge

The simple nuclear model of the atom: the nucleus consists of positive protons and uncharged neutrons. It’s surrounded by negative, orbiting electrons. The nucleus of the atom is much smaller than the size of the atom itself. 

  1. Proton umber: the number of protons in the nucleus
  2. Nucleon number: the number of protons and neutrons (nucleons) in the nucleus
  3. Isotopes: nuclei of the same element with the same number of protons but different numbers of neutrons

The notation represents nuclei, where X is the chemical symbol, A is the nucleon number and Z is the atomic number. The masses of atoms and nuclear particles are often expressed as atomic mass units, u. One atomic mass unit is one twelfth of the mass of a neutral carbon-12 atom.

The radius of a nucleus is given by the equation:


r_{0}  is a constant and is the nucleon number. The mean density of nuclei (1017kgm-3) far exceeds that of                      atoms (103kgm-3). 

Protons are held together despite their mutual electrostatic repulsion by the strong nuclear force. This is a short-range force which is attractive to about 3fm and repulsive below about 0.5fm.