A water molecule is made up of two atoms of hydrogen and one atom of oxygen. Oxygen atom has six electrons in its outer most shell and a hydrogen atom has one electron in its outer most shell. Oxygen and hydrogen atoms are unstable because they have less than eight electrons in their outer most shells. Oxygen needs two electrons and hydrogen atom requires one electron to complete their valence shell. Oxygen atom combines with two hydrogen atoms by sharing electrons. The bonds present between hydrogen atoms and oxygen atom are called single covalent bonds. In this way, oxygen and hydrogen atoms complete their outer most orbits. Oxygen shares two electrons with hydrogen atoms but its four electrons remain free in the water molecule. Therefore, there are two electron pairs or four electrons present in a water molecule.