When atoms of similar IE or EA approach each other there is a decrease in potential energy as attractions may be experienced by electrons in one atom for the nucleus of the other. Assuming the valence level of at least one atom is not complete, it may be possible to arrive at an internuclear distance in which these attractions outweigh the repulsions of the electron "clouds" for one another. Under these circumstances covalent bonds may form as electron orbital probability maps overlap. Using orbital diagrams we could represent the situation for water.

Lewis notation was actually meant for this type of situation.

However we depict it the result is the same. Because energy is released when the bond forms, the bonded molecule is sometimes described as being at the bottom of an energy "well". The "bond energy" would have to be "poured back" into the well to float the molecule out and break the bond.