When atoms gain or lose electrons what do they become?
Ions - an ion is an atom or molecule which has gained or lost electrons.
Atoms and chemical species lose or gain electrons when they react in order to gain stability. Thus, typically, metals (with nearly empty outer shells) lose electrons to non-metals, thereby forming positive ions.
The number of electrons depends on their position on the Periodic table (in simple terms). for example, group I metals lose one electron to form +1 ions, group II lose 2 electrons to form +2 ions etc. non-metals gain electrons from metals in order to achieve full outer shells - GVII gain 1 to form 1- ions, GVI gain 2 to form 2- ions etc. Thus, metals will typically react with non-metals, exchanging electrons to form ionic compounds.