Formation of ions in ionic compounds (Groups 1, 2, 6 and 7)

Atoms form ions so that they have a full outer shell of electrons. The group number tells you which ion an atom forms. Group 1 atoms lose one electron to form 1+ ions, such as Na⁺. Group 2 atoms lose two electrons to form 2+ ions, such as Mg²⁺ and Ca²⁺. Metals in Groups 1 and 2 therefore form ions with a positive charge.

Each Group 6 atom gains two electrons to form a 2- ion, such as the oxide ion O^2- (2.6 becomes 2.8). Group 7 atoms gain one electron to form 1- ions, such as the chloride ion Cl^- (2.8.7 becomes 2.8.8). Non-metals in Groups 6 and 7 form ions with a negative charge. Magnesium, 2.8.2, loses two electrons to become Mg²⁺ with the configuration 2.8.

An ionic compound is electrically neutral, so the total positive charge must equal the total negative charge. Calcium oxide, CaO, has one Ca²⁺ for each O^2-. Magnesium chloride needs two Cl^- ions for every Mg²⁺ ion, so its formula is MgCl₂. Sodium oxide needs two Na⁺ ions for each O^2- ion, so its formula is Na₂O.