VSEPR and Bond Hybridization
Introduced| English |
|---|
| VSEPR/ˈvespə/ |
| hybridization/ˌhaɪbrɪdaɪˈzeɪʃn/ |
Why molecules have shapes
- Molecules are not flat blobs -- they have definite 3D shapes.
- Water is bent; carbon dioxide is a straight line.
- One simple idea predicts every shape.
- Electron pairs push apart as far as they can.
Electron pairs spread out
- VSEPR 价层电子对互斥 says electron pairs repel and spread as far apart as possible.
- Count the groups (bonds and lone pairs) around the central atom.
- Their arrangement sets the shape.
VSEPR predicts shape by assuming electron groups...
Repelling groups spread out, which fixes the geometry.
The common shapes
- 2 groups give linear ($180^\circ$); 3 groups give trigonal planar ($120^\circ$).
- 4 groups give tetrahedral ($109.5^\circ$).
- Lone pairs push harder, bending the shape -- water comes out bent.
What is the shape of $\text{CH}_4$ (4 bonds, no lone pairs)?
Four groups spread to a tetrahedron at $109.5^\circ$.
Water ($\text{H}_2\text{O}$) has 2 bonds and 2 lone pairs. Its shape is...
The two lone pairs push the two bonds into a bent shape.
A central atom with exactly 2 electron groups has a ____ shape.
Two groups point opposite ways, $180^\circ$ apart -- linear.
Hybridization matches the geometry
- Hybridization 杂化 mixes atomic orbitals to match the shape.
- 2 groups give sp; 3 groups give sp$^2$; 4 groups give sp$^3$.
- Just count the groups and read off the label.
Predict the shape
Count bonding pairs and lone pairs, then read off the VSEPR shape.
Match the number of electron groups to the hybridization.
Hybridization counts groups, so 2 = sp, 3 = sp$^2$, 4 = sp$^3$.
Predict the shape of $\text{CH}_4$ and $\text{H}_2\text{O}$.
- $\text{CH}_4$: 4 bonds, no lone pairs, so tetrahedral and sp$^3$.
- $\text{H}_2\text{O}$: 2 bonds plus 2 lone pairs, so bent, still sp$^3$.
Lone pairs must be counted as electron groups when predicting shape.
Lone pairs occupy space and bend the geometry.
Count lone pairs as electron groups -- they take up space and bend the shape, even though they do not appear in the shape's name. Water is "bent," not "linear," because of its two lone pairs. Hybridization follows the number of groups, not the number of bonds alone.
VSEPR predicts shape by spreading electron groups as far apart as possible: 2 groups linear, 3 trigonal planar, 4 tetrahedral, with lone pairs bending the result. Hybridization matches the count -- sp, sp$^2$, sp$^3$ for 2, 3, 4 groups. Always count lone pairs as groups.