The smallest part of an element that can exist.
Atoms consist of a small dense nucleus of protons and neutrons surrounded by moving electrons.
The number of electrons equals the number of protons so the overall charge is zero. The electronic structure of an atom refers to the way in which the electrons are arranged about the nucleus, and in particular the energy levels that they occupy. Traditionally, these levels, or the orbits corresponding to them, are referred to as shells and given letters K, L, M, etc. The K-shell is the one nearest the nucleus. The maximum number of electrons in a given shell is 2 n 2 .