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).