A molecule is triatomic when it consists of three atoms.Water is an example of a triatomic molecule consisting of two hydrogen atoms and one oxygen atom combined to form a single molecule. As these atoms combine, there is shifting of valence electrons, i.e., the atoms in the outside electron shell, of each atom. When triatomic molecules are formed, the outer energy level of each atom is usually completed, either as the result of electron pair sharing or of the loss or gain of electrons.