---
title: "Lone Pair — AP Chem Definition, VSEPR & Geometry Guide"
description: "A lone pair is a valence electron pair on an atom that isn't shared in a bond. Lone pairs drive VSEPR geometry, squeeze bond angles, and decide polarity."
canonical: "https://fiveable.me/ap-chem/key-terms/lone-pair"
type: "key-term"
subject: "AP Chemistry"
unit: "Unit 2"
---

# Lone Pair — AP Chem Definition, VSEPR & Geometry Guide

## Definition

A lone pair is a pair of valence electrons on an atom that is not shared in a covalent bond. In AP Chem (Topic 2.7), lone pairs count as electron domains in VSEPR theory, so they shape molecular geometry and compress bond angles because they repel bonding pairs more strongly.

## What It Is

A lone pair is a set of two [valence electrons](/ap-chem/unit-1/valence-electrons-ionic-compounds/study-guide/XTtinEfGPR0jEJmpUuBx "fv-autolink") that sits on one atom instead of being shared between two atoms in a bond. You find lone pairs by drawing a Lewis diagram. Any valence electrons on the [central atom](/ap-chem/key-terms/central-atom "fv-autolink") that aren't part of a bond are lone pairs.

Here's why they matter so much. [VSEPR theory](/ap-chem/unit-2/vsepr-bond-hybridization/study-guide/OslsAmh8LcVoqbpnjPAu "fv-autolink") (EK 2.7.A.1) says electron pairs around a central atom repel each other through Coulombic repulsion and spread out as far as possible. Lone pairs count as electron domains just like bonds do, but they're held by only one nucleus, so they take up more space and push harder. That extra push is what turns a tetrahedral electron arrangement into trigonal pyramidal (one lone pair, like NH₃) or bent (two lone pairs, like H₂O), and it's why bond angles shrink below the ideal values. Lone pairs are invisible in the molecule's shape name but loud in its geometry.

## Why It Matters

Lone pairs live in [Unit 2](/ap-chem/unit-2 "fv-autolink") (Compound Structure and Properties), specifically Topic 2.7, and they sit at the heart of learning objective 2.7.A. That LO asks you to use Lewis diagrams and VSEPR together to explain a molecule's structural and electronic properties, including all the geometries listed in EK 2.7.A.2 (linear, trigonal planar, [tetrahedral](/ap-chem/key-terms/tetrahedral "fv-autolink"), trigonal pyramidal, bent, trigonal bipyramidal, seesaw, T-shaped, octahedral, square pyramidal, square planar). Almost every geometry in that list beyond the basic five exists *because* of lone pairs. Seesaw is trigonal bipyramidal with one lone pair. Square planar is octahedral with two. If you can't count lone pairs from a Lewis structure, you can't name the shape, predict the bond angle, or decide whether the molecule is polar, and all three of those show up on the exam.

## Connections

### [Molecular Geometry (Unit 2)](/ap-chem/key-terms/molecular-geometry)

Geometry names describe where the atoms are, not where the [electrons](/ap-chem/unit-1/atomic-structure-electron-configurations/study-guide/DiW6kVmwDRDakxKodjw5 "fv-autolink") are. Lone pairs occupy positions in the electron arrangement but get dropped from the shape name, which is why H₂O has four electron domains but is called bent, not tetrahedral.

### Molecular polarity and dipole moment (Unit 2)

Lone pairs often break a [molecule](/ap-chem/unit-2/lewis-diagrams/study-guide/KjqTRYr5TVr2C3Be3u0J "fv-autolink")'s symmetry, so bond dipoles stop canceling. NH₃ is polar largely because its lone pair makes the shape pyramidal instead of flat. The exception to know is square planar XeF₄, where two lone pairs sit opposite each other and symmetry survives.

### [sp3 hybridization (Unit 2)](/ap-chem/key-terms/sp3-hybridization)

Hybridization is counted from total electron domains, and lone pairs count. The oxygen in water is sp³ even though it only bonds to two hydrogens, because its two lone pairs fill the other two hybrid [orbitals](/ap-chem/key-terms/orbitals "fv-autolink").

### [Bond polarity (Unit 2)](/ap-chem/key-terms/bond-polarity)

Bond polarity tells you each bond's individual dipole, but lone pairs decide the 3D arrangement those dipoles sit in. You need both to call a molecule polar or nonpolar, which is exactly the Lewis-plus-VSEPR pairing LO 2.7.A demands.

## On the AP Exam

Multiple-choice questions love making you count electron domains and lone pairs from a formula, then match the result to a geometry, hybridization, and bond angle. A classic stem gives you a central atom with 4 domains and 0 lone pairs and asks for the full geometry-hybridization-angle combo. Harder versions hand you XeF₄ and ask you to explain why it's square planar (two lone pairs occupying opposite positions in an octahedral arrangement), or ask you to rank PCl₅, SF₄, and ClF₃ by deviation from ideal bond angles (more lone pairs means more squeeze, so more deviation). On FRQs, lone pairs appear inside Lewis-structure and geometry questions about real molecules like the nitrogen oxides in the 2018 long FRQ. The move you must make every time is the same. Draw the Lewis diagram, count bonding domains and lone pairs separately, and use lone-pair repulsion as the *reason* in your explanation, not just the answer.

## lone pair vs bonding pair

Both are pairs of valence electrons and both count as electron domains in VSEPR, but a bonding pair is shared between two nuclei while a lone pair belongs to one atom alone. Because a lone pair is pulled by only one nucleus, its electron cloud spreads wider and repels neighbors more. The repulsion order is lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair, which is why lone pairs compress bond angles (water's angle is about 104.5°, not the ideal 109.5°).

## Key Takeaways

- A lone pair is a pair of valence electrons on an atom that is not shared in a bond, and you find them by drawing the Lewis diagram.
- Lone pairs count as electron domains in VSEPR, so they help determine the electron arrangement and the hybridization of the central atom.
- Lone pairs repel more strongly than bonding pairs, which compresses bond angles below ideal values, like water's 104.5° instead of 109.5°.
- The molecular geometry name only describes atom positions, so adding lone pairs changes the name (tetrahedral becomes trigonal pyramidal, then bent).
- Lone pairs often make molecules polar by breaking symmetry, but symmetric lone-pair placement, as in square planar XeF₄, can keep a molecule nonpolar.
- On the exam, always count bonding domains and lone pairs separately, then use lone-pair repulsion as your written justification for geometry and angle answers.

## FAQs

### What is a lone pair in AP Chemistry?

A lone pair is a pair of valence electrons that sits on one atom and isn't shared in a covalent bond. In Topic 2.7, lone pairs count as electron domains in VSEPR theory and determine molecular geometry, bond angles, and polarity.

### Do lone pairs count in molecular geometry?

They count toward the electron arrangement but not the shape name. Water has four electron domains (two bonds, two lone pairs), so its electron geometry is tetrahedral, but its molecular geometry is bent because the name only describes where atoms sit.

### Why do lone pairs decrease bond angles?

A lone pair is attracted to only one nucleus, so its electron cloud spreads out more and repels bonding pairs harder. That stronger Coulombic repulsion squeezes the bonds together, dropping NH₃ to about 107° and H₂O to about 104.5° from the ideal 109.5°.

### What's the difference between a lone pair and a bonding pair?

A bonding pair is shared between two atoms; a lone pair belongs to just one. Both count as electron domains, but lone pairs repel more strongly, following the order lone-lone > lone-bonding > bonding-bonding.

### Does a molecule with lone pairs have to be polar?

No. Lone pairs usually break symmetry and create polarity (like NH₃ and H₂O), but if they're placed symmetrically, the molecule can stay nonpolar. XeF₄ is the classic exam example, with two lone pairs opposite each other giving a nonpolar square planar shape.

## Related Study Guides

- [2.7 VSEPR and Bond Hybridization](/ap-chem/unit-2/vsepr-bond-hybridization/study-guide/OslsAmh8LcVoqbpnjPAu)

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