An atom has a very small nucleus at its centre. The nucleus contains two types of particle, protons and neutrons. Protons are positively charged and neutrons are neutral, so the nucleus has an overall positive charge.
Electrons are negatively charged and move around the nucleus in shells, also called energy levels. They are attracted to the positive nucleus. Electrons fill the shell closest to the nucleus first, and this first shell holds a maximum of two electrons. Once it is full, the next electron goes in the second shell.
