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:
- Most of the alpha particles passed straight through the gold foil with very little scattering, so the atom must be mostly empty space
- 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.
- Proton umber: the number of protons in the nucleus
- Nucleon number: the number of protons and neutrons (nucleons) in the nucleus
- 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:
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.