Hybridization: MDCAT Chemistry notes
Hybridization for MDCAT: sp3, sp2 and sp orbitals, s-character, shape and size of hybrids, and identifying hybridization in molecules and groups.
What hybridization is
Hybridization is the mixing of atomic orbitals of nearly equal energy on the same atom to form an equal number of new, identical hybrid orbitals. The number of hybrid orbitals always equals the number of atomic orbitals mixed, so mixing 4 orbitals gives 4 hybrids. Hybrid orbitals overlap head-on, so they form only sigma (σ) bonds or hold lone pairs. Pi bonds come from the leftover, unhybridized p orbitals.
The three types for carbon and similar atoms
| Type | Orbitals mixed | s character | Geometry, angle | Examples |
|---|---|---|---|---|
| $sp^3$ | 1 s + 3 p | 25% | Tetrahedral, 109.5° | $\mathrm{CH_4}$, ethane, all alkanes, diamond, $\mathrm{NH_3}$ and $\mathrm{H_2O}$ (with lone pairs) |
| $sp^2$ | 1 s + 2 p | 33% | Trigonal planar, 120° | $\mathrm{BF_3}$, $\mathrm{BH_3}$, ethene, graphite, C=O carbon |
| $sp$ | 1 s + 1 p | 50% | Linear, 180° | $\mathrm{BeCl_2}$, ethyne, $\mathrm{CO_2}$ carbon, nitrile (−C≡N) carbon |
sp2 hybridization in detail (ethene)
Each carbon in $\mathrm{H_2C{=}CH_2}$ forms three σ bonds (two C–H, one C–C) using three $sp^2$ orbitals. These orbitals are coplanar and lie at 120° to one another. The remaining p orbital is perpendicular to the plane of the $sp^2$ orbitals. It overlaps sideways with the p orbital on the other carbon to form the π bond.
Quick rule for finding hybridization
Count the σ bonds + lone pairs on the atom: 4 means $sp^3$, 3 means $sp^2$ and 2 means $sp$. For carbon, the bond types give it directly:
- Four single bonds: $sp^3$
- One double bond: $sp^2$
- One triple bond, or two double bonds: $sp$
Functional groups: in −CHO, −COOH and −COOR the carbon has a C=O double bond, so it is $sp^2$. In −CN the carbon has a C≡N triple bond, so it is $sp$.
Central atoms with three bonds and a lone pair, such as $\mathrm{NH_3}$, $\mathrm{PH_3}$ and $\mathrm{SbH_3}$, are pyramidal, not $sp^2$ planar. $\mathrm{BH_3}$ has no lone pair, so it is $sp^2$.
Size and s character
s orbitals are held closer to the nucleus than p orbitals. More s character therefore makes a hybrid smaller and more electronegative.
- Size: $sp^3 > sp^2 > sp$
- s character and electronegativity: $sp > sp^2 > sp^3$
- Bond angle: $sp$ (180°) > $sp^2$ (120°) > $sp^3$ (109.5°)
Common MDCAT traps
- The size order is the reverse of the s-character order. sp is the smallest.
- The unhybridized p orbital in $sp^2$ is perpendicular to the plane, not in it or parallel to it.
- In ethene, the carbon is $sp^2$ even though the molecule also contains a π bond.
- Diamond carbon is $sp^3$ and graphite carbon is $sp^2$.
- $\mathrm{BF_3}$ and $\mathrm{BH_3}$ are $sp^2$. $\mathrm{NH_3}$ is not, because of its lone pair.
Quick revision
- n atomic orbitals mixed give n hybrid orbitals.
- Hybrid orbitals form σ bonds only.
- sp has the maximum s character (50%).
- All alkane carbons are $sp^3$ and nitrile carbon is $sp$.
- $sp^2$ orbitals are coplanar at 120°.