* Metals have low electronegativity: They have a weaker hold on their valence electrons.
* Nonmetals have high electronegativity: They have a strong attraction for electrons.
When a metal and a nonmetal bond ionically, the metal atom *loses* one or more electrons to become a positively charged ion (cation). The nonmetal atom *gains* those electrons, becoming a negatively charged ion (anion).
Example: In sodium chloride (NaCl), sodium (Na) is a metal and chlorine (Cl) is a nonmetal. Sodium loses an electron to become a sodium cation (Na+), while chlorine gains that electron to become a chloride anion (Cl-). The opposite charges attract each other, forming the ionic compound.
