Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The electrons are arranged in energy levels, or shells, around the nucleus.
The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall. Atoms can lose or gain electrons. When they do, they form charged particles called ions:
if an atom loses one or more electrons, it becomes a positively charged ion
if an atom gains one or more electrons, it becomes a negatively charged ion