Hybridization (Part of VBT) The 2s and 2p orbitals are similar in energy. Recall that they are identical for hydrogen. Combining the wavefunctions for 2s and 2p gives a solution to the Schrödinger equation. The s orbital is combined with p orbitals – the number of p orbitals used depends on the molecule to be formed. The resulting (new) atomic orbitals are hybridized orbitals. A molecule is not simply atoms stuck together.

Methane (CH4) C: 1s22s22p2 C*: 1s22s12p3 The s and p orbitals form 4 equivalent hybrid orbitals made from linear combinations of the atomic carbon orbitals. These give the correct geometry.

Hybridization and VSEPR These are different models but they are consistent. Ammonia Tetrahedral geometry (like methane). sp3 hybridized (like methane).

Ethylene (H2C=CH2) We see a trigonal planar geometry (and shape), which means 120° angles (not sp3). With three effective pairs, we need three hybrid orbitals. One s and two p to give three sp2 hybrid orbitals. This leaves one p orbital unhybridized.

Ethylene (H2C=CH2)

Ethylene (H2C=CH2)

Carbon dioxide (O=C=O) Linear geometry and shape (180°). With two effective pairs, we need two hybrid orbitals. One s and one p to give two sp hybrid orbitals. This leaves two p orbitals unhybridized.

Carbon dioxide (O=C=O) Oxygen atoms are sp2 hybridized (two lone pairs).

Overview Conservation of orbitals: total of 4 in all cases.

CO (:C≡O:) is like N2 (:N≡N:) We use the total number of electrons available in the molecule (as we did with Lewis structures).

Breaking the octet? Use d orbitals? PCl5 SF6

Hybridization and VSEPR

Hybridization Pros Cons Molecules do not just consist of atoms stuck together (something “happens” to make a molecule different from the atoms). Relatively straightforward. Pictures! Wait until MO Theory… Easy to apply VSEPR. Cons Still cannot explain odd number of electrons. Bond energies not explained Ions like O2+. Magnetism not explained. Colors of solutions not explained.

Molecular Orbital Theory Molecule: Simplest: overlap of unchanged atomic orbitals (atoms stuck together) Hybridization: overlap of hybridized atomic orbitals localized between nuclei MO Theory: collection of nuclei with delocalized molecular orbitals Linear combination of atomic orbitals in the molecule to form molecular orbitals. Atomic orbitals must overlap to form molecular orbitals. Wave functions can constructively and destructively interfere. Conservation of the number of orbitals – the number of molecular orbitals equals the total number of molecular orbitals.