When a metal atom reacts with a non-metal atom, electrons from the outer shell of the metal atom are transferred to the non-metal atom. The metal atom loses electrons and becomes a positively charged ion. The non-metal atom gains electrons and becomes a negatively charged ion. This is ionic bonding.
The ions formed by metals in Groups 1 and 2, and by non-metals in Groups 6 and 7, have the electronic structure of a noble gas (Group 0). They have a full outer shell. For example, a sodium atom (2,8,1) transfers one electron to a chlorine atom (2,8,7). The sodium ion is 2,8 and the chloride ion is 2,8,8.
A dot and cross diagram shows this transfer. The outer electrons of one atom are drawn as dots and those of the other atom as crosses, so you can see where each electron ends up.
The charge on the ion comes from the group number. Group 1 metals form ions with a charge of 1+ and Group 2 metals form ions with a charge of 2+. Group 7 non-metals form ions with a charge of 1- and Group 6 non-metals form ions with a charge of 2-. In magnesium oxide, each magnesium atom loses two electrons to form Mg2+ and each oxygen atom gains two electrons to form O2-, so both ions have a full outer shell.
