An ion is an atom or group of atoms with a positive or negative charge. The atoms of most elements have incomplete valance shells (outer shells) of electrons and want to achieve a full valence shell in order to be stable. This means they can lose or gain electrons during chemical reactions. When this happens, the atoms become ions. Groups of atoms may also become ions if they lose or gain electrons.
Metal atoms lose electrons to form positively charged ions called cations. For example, sodium atoms - Na - become sodium ions - Na+. The + sign shows that a sodium ion carries a single positive charge. Metal atoms lose outermost electrons when they form ions. For elements in groups 1 and 2, the number of outer electrons lost is the same as the group number.
Non-metal atoms gain electrons to form negatively charged ions called anions. For example, chlorine atoms, Cl, become chloride ions, Cl-. The - sign shows that a chloride ion carries a single negative charge. The end of the name changes to 'ide' when non-metal atoms form ions. Non-metal atoms gain electrons to complete their outermost shell when they form ions. Elements in groups 5, 6 and 7 all have negative charges of -3, -2 and -1.
When atoms of different elements combine to form compounds, new chemical bonds form. For example, when sodium chloride forms, electrons are transferred from sodium atoms to chlorine atoms, forming Na+ ions and Cl- ions. These oppositely charged ions attract each other strongly. New chemical bonds form, called ionic bonds.