Molecular shape and polarity
A molecule's shape determines its polarity, which in turn controls whether it dissolves, how it boils, and how it interacts with biological molecules.
VSEPR theory
Valence Shell Electron Pair Repulsion (VSEPR) theory says electron groups around a central atom (bonds and lone pairs) spread out as far apart as possible. Count the electron groups: 2 gives linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral arrangements.
Lone pairs change the shape
Lone pairs occupy space but are not counted when naming the shape. Methane (\(\text{CH}_4\)) has 4 bonds and is tetrahedral with \(109.5^\circ\) angles. Ammonia (\(\text{NH}_3\)) has 3 bonds and 1 lone pair: trigonal pyramidal, about \(107^\circ\). Water has 2 bonds and 2 lone pairs: bent, about \(104.5^\circ\).
Multiple bonds in VSEPR
A double or triple bond counts as a single electron group. Carbon dioxide has two double bonds and no lone pairs on carbon, so it is linear (\(180^\circ\)). Formaldehyde (\(\text{H}_2\text{CO}\)) has three electron groups around carbon and is trigonal planar.
Molecular polarity
A molecule is polar if its bond dipoles do not cancel. \(\text{CO}_2\) has polar bonds but is nonpolar because its linear shape cancels the dipoles. Water's bent shape leaves a net dipole, making it polar. Shape and bond polarity together decide molecular polarity.
Hybridisation
Valence bond theory describes bonding with hybrid orbitals: \(sp\) for linear, \(sp^2\) for trigonal planar, \(sp^3\) for tetrahedral arrangements. Carbon in methane is \(sp^3\); in ethene (\(\text{C}_2\text{H}_4\)), each carbon is \(sp^2\), with a pi bond above and below the plane forming part of the double bond.
Sigma and pi bonds
A single bond is one sigma bond, formed by head-on orbital overlap. A double bond is one sigma plus one pi bond (side-on overlap); a triple bond is one sigma plus two pi bonds. Pi bonds prevent rotation, which is why double-bonded molecules can have fixed cis and trans arrangements.
Worked example
Predict the shape of \(\text{SF}_6\). Sulfur has six bonds and no lone pairs: six electron groups, octahedral shape, \(90^\circ\) angles. Although each S–F bond is polar, the symmetric shape makes the molecule nonpolar. Sulfur here holds 12 electrons, an expanded octet possible for period 3 and beyond.
Why polarity matters
Polar molecules dissolve in polar solvents ('like dissolves like'), which is why salt dissolves in water but oil does not. Polarity also affects boiling points and how drugs bind to proteins, making shape central to chemistry and biology.
Key takeaways
VSEPR predicts shapes from the number of electron groups; lone pairs compress angles and change the shape's name. Multiple bonds count as one group. Polarity depends on both bond polarity and shape. Hybrid orbitals and sigma and pi bonds describe bonding in more detail.
Flashcard
Electron groups repel to maximise separation
Check your understanding
Why is CO2 nonpolar even though its C=O bonds are polar?