Although there are only 112 known elements, there are millions of chemical substances found in nature or made artificially. These substances are not simply mixtures of two or more elements, but chemical compounds, formed by combining
two or more elements together in a chemical reaction. The chemical ‘glue’ that holds compounds together is called chemical bonding. Chemical bonding also holds together atoms in the molecules of an element. There are two main types of chemical bond: covalent and ionic. Both work by bringing two atoms together in such a way as to make them appear stable. This is achieved by interaction in the atoms’ outermost shell of electrons (valence shell). By either sharing or donating electrons, both atoms can fill up their valence shell with electrons and achieve a stability similar to the noble gases.
Covalent bonding occurs when two atoms share electrons in their outer shell. Fluorine atoms, for example, each have seven outer electrons, but to gain a stability they need eight outer electrons. Thus two fluorine atoms can share an electron each, forming a covalent bond.
Ionic bonding (or electrovalent bonding) occurs when one atom ‘donates’ an electron to another atom. For example, sodium (Na) has one electron in its outer shell, whilst fluorine has seven. If the sodium atom transfers an electron to fluorine, both atoms attain a stability in their outer shell. When this transfer happens, both atoms become ions.