Notice that the lone pairs of electrons on the four chlorine atoms are not shown but are understood to be there. Since the majority of the bonds we will encounter in organic chemistry are covalent bonds, we need to look at the possible ways of explaining how covalent bonds are formed. There are two models which explain covalent bonding, Valence Bond Theory and Molecular Orbital Theory. In Valence Bond Theory, bonding is viewed as occurring by the overlap of two atomic orbitals, one from each atom. Each atom's orbital contains a single electron and a bond is formed by the electrons, now paired in overlapping orbitals, holding the two nuclei together.
Every covalent bond has a characteristic bond strength and bond length.
The bond length is defined as the distance between the two nuclei. In Molecular Orbital Theory, bonding is explained in terms of the mathematical combination of atomic orbitals to form molecular orbitals. The newly formed orbitals are called molecular orbitals because they belong to the entire molecule. The combination of two atomic orbitals leads to two molecular orbitals, a bonding molecular orbital and an antibonding molecular orbital.