Orbital Hybridization

What is orbital hybridization?

The application of quantum mechanics to the simple hydrogen-electron model which allows for the creation of more complex electron distribution models, i.e. hybridized orbitals.

In organic chemistry, the main atom we will look at is carbon. The orbitals that are hybridized are $s$ and $p$, which `coalesce' to form new orbitals.

What are three levels of orbital hybridization seen in carbon bonds?

$sp^{3}$	Single bonds
$sp^{2}$	Double bonds
$sp$	Triple bonds

What are the geometries of these three levels of orbital hybridization?

$sp^{3}$	Tetrahedral	4 $e^{-}$ pairs
$sp^{2}$	Triangular planar	3 $e^{-}$ pairs
$sp$	Linear	2 $e^{-}$ pairs

See Figure 40.1 below.

Figure 40.1: Bond geometries: (Left) Tetrahedral, note the 109.5$^{o} \angle$, (center) triangular planar, note the 120$^{o} \angle$, (right) linear, note the 180$^{o} \angle$.

What is the valence shell electron repulsion theory (VSEPR)?

A theory which predicts the shape of a molecule by taking into account all pairs of bonding and non-bonding electrons. By considering all pairs of electrons - and the fact that electrons repel each other - approximate geometries of molecules can be predicted:

Table 40.1: Valence shell electron repulsion theory (VSEPR)

Bond angle	# of
geometry ($^{o}$)	$e^{-}$ pairs	Molecular geometry
Linear (180$^{o}$)	2	Linear	CO$_{2}$
Trigonal planar (120$^{o}$)	3	Trigonal planar	BH$_{3}$
Tetrahedral (109.5$^{o}$)	4	Tetrahedral	CH$_{4}$
Tetrahedral (107$^{o}$)	4	Trigonal pyramidal	NH$_{3}$
Tetrahedral (105$^{o}$)	4	Bent	H$_{2}$O
Trigonal bipyramidal (120$^{o}$ & 90$^{o}$)	5	Trigonal bipyramidal	PCl$_{5}$
Octahedral (90$^{o}$)	6	Octahedral	SF$_{6}$

Note: In some molecules, the molecular geometry differs from the bond angle geometry because lone electron pairs are not taken into account for the molecular geometry. Ammonia and water exemplify this well this because the lone electron pairs push the surrounding atoms closer together, making for narrower bond angles (107$^{o}$ or 105$^{o}$) than what would be expected for a tetrahedral bond arrangement between four bonding atoms (109.5$^{o}$).

Figure 40.2: Selected molecular geometries: hydrogen (H), silicon (Si), phosphorus (P), oxygen (O), chloride (Cl), carbon (C), nitrogen (N), sulfur (S), fluorine (F).

Between non-bonding or bonding pairs of electrons - which occupies more space?

Non-bonding pairs of electrons take up more space.

What are the electron dot structures for H, C, N, O, F, S, P, Si, and Cl?

Figure 40.3: Selected electron dot structures: (a) H, (b) C, (c) N, (d) O, (e) F, (f) S, (g) P, (h) Si, and (i) Cl.

What are resonance structures?

Two or more structures that can correctly represent the electron distribution of a molecule. Molecules which have resonance structures, e.g. H$_{2}$O, never really exist in one form or another. Instead, they are constantly in flux, oscillating between resonance structures, see Figure 40.4 below:

Figure 40.4: Resonance structures of H$_{2}$O.

In covalent bonds, what is the relationship between bond length and bond energy?

Bond length and bond energy are inversely related: the shorter the bond the greater the bond energy.