An atom that has lost electrons has a positive charge. These atoms are now called ions, more specifically cations. In general, cations are electropositive elements that hang onto their electrons very loosely.
Atoms in their neutral state have the same number of electrons and protons. Electrons have negative charges, whereas protons have positive charges. If an atom loses negative charges, it gains a net positive charge.
Some atoms hang onto their electrons tighter than others, and others give them up readily. Metallic elements, such as sodium, potassium and magnesium, are electropositive elements that easily form cations. These atoms have one or two electrons in their outermost energy levels and tend to donate them to electronegative elements, such as oxygen and fluorine.