Electrons
It's not neutrons because neutrons don't have a charge (they're not positive or negative). They don't make a difference in the charge of an atom.
It's not protons because atoms don't lose or gain protons. They stay in the nucleus.
Electrons, however, are not in the nucleus. They fly crazy fast around the nucleus and can sometimes get sucked in by the nucleus of another atom. Electrons are also negatively charged. Gaining electrons makes an atom negatively charged. Losing electrons makes an atom positively charged.
