There is more number of substances in the universe than 109 elements. Most matter we run into on a day-to-day basis is composed of compounds-substances in which two or more elements have bonded together to form a material with unique chemical properties. One of the like compounds is water. A water molecule consists of two atoms of hydrogen bonded to one atom oxygen. Thus what is chemical bonding and why does it occur?
If we ignore the transition metals in the periodic table, it becomes apparent that every electron shell beyond the first has a total capacity for 8 valance (outer shell) electrons (the first shell has a capacity for 2 valance electrons). We can represent the actual number of electrons in an atom's valance shell by drawing each valance electron as a dot surrounding the element symbol. This is called the Lewis dot structure and some example follow:
As in turns out, atoms bond together for a very simple reason: atoms like to contain full valance shells. Most of the materials we see around us are not composed of just single specific of atoms. They are compounds, composed of molecules made up of atoms from two or more elements. The two or more atoms combine to form molecules of a compound and thus the interaction occurs in different ways between different kinds of atoms and lead to the formation of variety of bonds. The primary atomic bonds include the ionic, covalent, and metallic bonds.