Answer: The correct answer for the fill in the blank is-  

An ion is formed when an atom gain/ loses an electron.

Example- When Sodium atom loses one electron, it becomes cation ( Na⁺)

Atoms are the fundamental unit of matter, which are neutral ( that is lack charge) and composed of three subatomic particles ( electron, proton, and neutron).

An ion is an electrically charged atom that is formed when atom loses or gains an electron.

There are two types of ion that is- Anion (negatively charged, produced by gain of electron) and cation ( positively charged, produced by loss of electron).