What are ions?

An ion is simply an atom or molecule with a positive or negative charge. Such a charge results from an uneven number of protons and electrons within the atom or molecule.

All atoms consist of a nucleus, composed of positively-charged protons and unchanged neutrons, surrounded by negatively charged electrons. Under normal conditions, the number of protons and electrons will be equal. Since the charges due to electrons and those due to protons are equal and opposite, the net charge is zero.

However, should the atom gain or lose one or more electrons, the charges will no longer balance. The atom will now have either more protons than electrons or vice-versa, resulting in an overall positive or negative charge. The atom is then subject to attraction or repulsion due to electomagnetic fields: this is the basis of ion propulsion.