atom, electronic structure of

Arrangement of electrons around the nucleus of an atom, in distinct energy levels, also called atomic orbitals, or shells. In a simplified description, these shells can be regarded as a series of concentric spheres, each of which can contain a certain maximum number of electrons; the noble gases (rare gases) have an arrangement in which every shell contains this number (see noble gas structure). The energy levels are usually numbered beginning with the shell nearest to the nucleus. The outermost shell is known as the valency shell as it contains the valence electrons.

The lowest energy level, or innermost shell, can contain no more than two electrons. Outer shells are considered to be stable when they contain eight electrons but additional electrons can sometimes be accommodated provided that the outermost shell has a stable configuration. Electrons in unfilled shells are available to take part in chemical bonding, giving rise to the concept of valency. In ions, the electron shells contain more or fewer electrons than are required for a neutral atom, resulting in negative or positive charges.

The atomic number of an element indicates the number of electrons in a neutral atom. From this it is possible to deduce its electronic structure. For example, sodium has atomic number 11 (Z = 11) and its electronic arrangement (configuration) is two electrons in the first energy level, eight electrons in the second energy level and one electron in the third energy level – generally written as 2.8.1. Similarly for sulphur (Z = 16), the electron arrangement is 2.8.6. The electronic structure dictates whether two elements will combine by ionic or covalent bonding (see bond) or not at all.