Nuclear structure

Nuclei consist of positively charged protons and uncharged neutrons; these two particles with about similar mass are known as nucleons. The number of protons is atomic number of an element (Z), and is matched in a neutral atom by similar number of electrons. The total number of nucleons is mass number and is sometimes specified through a superscript on the symbol of the element. So ¹H has a nucleus with one proton and no neutron, ²⁰⁸Pb has 82 protons and 126 neutrons, ¹⁶O has eight protons and eight neutrons. Neutrons and Protons are held together by an attractive force of very short range called the strong interaction. Opposite to this is the longer-range electrostatic repulsion among protons. The balance of the two forces controls some significant characteristics of nuclear stability.

1. Whereas lighter nuclei are usually stable with approximately equal numbers of neutrons and protons, heavier ones have a progressively higher proportion of neutrons (for example compare ¹⁶O with ²⁰⁸Pb).
2. As Z increments the electrostatic repulsion comes to dominate, and there is a limit to the number of stable nuclei, all elements further than Bi (Z=83) being radioactive.

As with electrons in atoms, it is essential to make use of the quantum theory to account for the details of nuclear stability and structure. It is favorable to 'pair' nucleons so that nuclei with even numbers of either protons or neutrons (or both) are usually more stable than ones with odd numbers. The nuclei's shell model, analogous to the orbital picture of atoms also predicts specific magic numbers of protons or neutrons, which give additional stability. These are as follows:

 2         8          20        28        50        82        126

¹⁶O and ²⁰⁸Pb are instances of nuclei with magic numbers of both neutrons and protons.

Trends in the stability of nuclei are significant not only in determining the number of elements and their isotopes but also in controlling the proportions in which they are made by nuclear reactions in stars. These determine the elements' abundance in the Universe as a whole.