There is a problem with basic valence bond theory We would expect that an atom could form a bond for every unpaired electron that is has in its valence orbitals. This is basically true, but consider an atom like beryllium. To understand why this is, we have to introduce hybrid orbitals . In order to form more bonds, the beryllium atom will hybridize, or mix, the 2s orbital and one of its 2p orbitals to form two 2sp hybrid orbitals.
Similar things happen in atoms with more electrons.
In the case of boron, the 2s and two 2p orbitals combine to form three 2sp 2 orbitals, and in carbon the 2s and all three 2p orbitals combine to four 2sp 3 orbitals.