lone pair
Summary
A lone pair is two valence electrons held on one atom and not shared in a bond. It counts in VSEPR geometry and makes an atom a Lewis base or nucleophile. Most testable: lone pairs repel more than bonding pairs, so bond angles shrink (NH₃ ≈ 107°, H₂O ≈ 104.5°).
Detail
Meaning: A lone pair (nonbonding pair) is two valence electrons held on one atom instead of shared in a bond. It counts as an electron domain in VSEPR and, held by only one nucleus, takes up more space than a bonding pair, so it compresses bond angles: NH₃ is about 107° and H₂O about 104.5°, against 109.5° for an ideal tetrahedron. Lone pairs on N, O and S make those atoms Lewis bases, nucleophiles and hydrogen-bond acceptors.
Equation: FC = V − L − B/2, where FC is formal charge (units of the elementary charge, 1.60 × 10⁻¹⁹ C), V is the valence electrons of the free atom, L is nonbonding electrons (2 per lone pair), and B is bonding electrons.
On the MCAT: Count lone pairs to predict shape, assign formal charge, start curved arrows at a lone pair, or rank basicity and nucleophilicity. An amine is more basic than an amide because the amide nitrogen's lone pair is delocalized by resonance into the C=O.
Common confusion: Molecular geometry names only the atoms, not the lone pairs: NH₃ has tetrahedral electron geometry but trigonal pyramidal molecular geometry. Also, a lone pair is not always available: a delocalized one, as in pyrrole's nitrogen, makes the atom a very weak base.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
- Organic Chemistry (Clayden, Greeves, Warren)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.