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The atomic nucleus is the densest part of any atom. It is the area that contains the protons and neutrons. In most atoms the nucleus accounts for over 99% of the atom’s mass. The electrons contribute very little to total atomic mass. The diameter of an atoms nucleus varies a great deal: a hydrogen nucleus, the lightest atom, is about 1.6 x 10-15m while the nucleus of the atoms of the heaviest elements can have a diameter of 15 x 10-15m. Nuclear physics is the field of study that concentrates on understanding the atomic nucleus.
The nucleus of an atom consists of protons and neutrons bound by residual strong force. Which element that an atom represents is determined by the number of protons in the nucleus. Each proton carries a single positive charge, and the total electrical charge of the nucleus is spread uniformly throughout its body, with a slight fall at the edge. Exceptions to this rule are the light elements hydrogen and helium, where the charge is concentrated most highly at a single central point. As each proton carries a unit of charge, the charge distribution is indicative of the proton distribution. The neutron distribution probably is similar.
There have been many atom models proposed to describe an atom’s nucleus. Several have placed the protons and neutrons(called nucleons) in orbit of each other. In these models, the nucleons may occupy orbitals in pairs, but the exact nature and capacity of nuclear shells differs from those of electrons in atomic orbitals, primarily because the potential well in which the nucleons move is quite different from the central electromagnetic potential well which binds electrons in atoms. There are; however, problems with the shell model you attempt to account for nuclear properties well away from closed shells. This as led to complex distortions of the shape of the potential-well to fit experimental data, but the question remains, do these mathematical manipulations actually correspond to the spatial deformations in real nuclei. Problems with the shell model have lead some to propose realistic two-and three-body nuclear force effects involving nucleon cluster, and then build the nucleus on this basis.
Discovering the atomic nucleus seems to be a little slippery. Many different formulations have been proposed and manipulated mathematically to suit a scientists needs. Here is a good article about the nucleus. Here on Universe Today we have a couple of great articles on the topic: one is about atomic mass and the other is about different atom models. Astronomy Cast offers a good episode about looking inside the atom.
Sources:
Wikipedia
Chemistry Explained
Windows to Universe
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