Difficulty: Intermediate | Prerequisites: Lewis structures, lone pairs, bond types (see Part 1)
VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the 3D shape of a molecule by assuming that electron groups around a central atom arrange themselves as far apart as possible. The shape you see depends on how many of those groups are bonds versus lone pairs. Count the electron domains, check how many are lone pairs, and you have the molecular geometry.
Electron domain (electron group)
Any region of electron density around a central atom. A single bond, a double bond, a triple bond, and a lone pair each count as one electron domain. In simple terms, whether it is a single bond or a triple bond, it is still one "group" pushing other groups away.
Electron-domain geometry
The arrangement of all electron domains (both bonding and lone pairs) around the central atom. This is the underlying spatial framework before you account for invisible lone pairs.
Molecular geometry (molecular shape)
The shape defined only by the positions of atoms, ignoring lone pairs. Because lone pairs are invisible in a physical model, the molecular geometry can differ from the electron-domain geometry.
AXE notation
A shorthand for describing molecular geometry. A = central atom, X = number of bonded atoms, E = number of lone pairs on the central atom. For example, AX₃E₁ means three bonded atoms and one lone pair.
Linear
180° bond angle. Two electron domains, or specific lone-pair arrangements (AX₂, AX₂E₃). Think of a straight line through the central atom.
Trigonal planar
120° bond angles. Three electron domains, all bonds, arranged in a flat triangle (AX₃).
Bent (V-shaped)
Occurs when lone pairs replace one or more bonding positions in a trigonal planar or tetrahedral arrangement. Bond angle is less than the ideal angle. Examples: H₂O, NOCl.
Tetrahedral
109.5° bond angles. Four electron domains, all bonds (AX₄). The classic example is CH₄.
Trigonal pyramidal
Three bonds and one lone pair around a tetrahedral electron-domain geometry (AX₃E₁). Bond angle is slightly less than 109.5° because the lone pair compresses the bonding angles. Example: NH₃ at about 107°.
Trigonal bipyramidal
Five electron domains around the central atom (AX₅). Has two types of positions: axial (top and bottom, 90° from equatorial) and equatorial (the three in the middle plane, 120° apart).
Seesaw (sawhorse)
Four bonds and one lone pair from a trigonal bipyramidal base (AX₄E₁). The lone pair occupies an equatorial position.
T-shaped
Three bonds and two lone pairs from a trigonal bipyramidal base (AX₃E₂). Both lone pairs sit equatorial.
Octahedral
Six electron domains, all at 90° from their neighbours (AX₆). Related shapes include square pyramidal (AX₅E₁) and square planar (AX₄E₂).
Expanded octet
When a central atom (Period 3 or below) has more than 8 valence electrons. Required for molecules like PCl₅, SF₄, ClF₃, and XeF₂.
The central idea: electron domains around a central atom repel one another electrostatically. The lowest-energy arrangement is the one where domains are as far apart as possible. Once you know the arrangement of all domains (the electron-domain geometry), you determine the molecular shape by looking only at the atoms, ignoring lone pairs.
Draw the Lewis structure.
Count the total electron domains on the central atom. Remember: a single bond, double bond, triple bond, or lone pair each count as one domain.
Determine the electron-domain geometry from the total count.
Note how many of those domains are lone pairs.
Name the molecular geometry based on the positions of the atoms only.
2 electron domains (linear arrangement)
AX₂: Linear, 180°. Example: BeCl₂, CO₂.
3 electron domains (trigonal planar arrangement)
AX₃: Trigonal planar, 120°. Example: BF₃.
AX₂E₁: Bent, less than 120°. Example: NOCl.
4 electron domains (tetrahedral arrangement)
AX₄: Tetrahedral, 109.5°. Examples: CH₄, NH₄⁺.
AX₃E₁: Trigonal pyramidal, about 107°. Example: NH₃.
AX₂E₂: Bent, about 104.5°. Example: H₂O.
5 electron domains (trigonal bipyramidal arrangement)
AX₅: Trigonal bipyramidal, 90° and 120°. Example: PCl₅.
AX₄E₁: Seesaw (sawhorse), less than 90° and less than 120°. Example: SF₄.
AX₃E₂: T-shaped, less than 90°. Example: ClF₃.
AX₂E₃: Linear, 180°. Example: XeF₂.
6 electron domains (octahedral arrangement)
AX₆: Octahedral, 90°. Example: SF₆.
AX₅E₁: Square pyramidal, less than 90°.
AX₄E₂: Square planar, 90°.
Lone pairs occupy more space than bonding pairs because they are held closer to the central atom (no second nucleus pulling them away). This extra repulsion squeezes the bonding pairs closer together, which is why NH₃ has a bond angle of about 107° rather than the ideal tetrahedral 109.5°, and H₂O is compressed further to about 104.5°.
Lone pairs preferentially occupy equatorial positions because equatorial slots have only two 90° neighbours, while axial slots have three. Placing the lone pair where it has fewer close neighbours minimises repulsion.
"Electron-domain geometry and molecular geometry are the same thing." They are the same only when all electron domains are bonds (no lone pairs). Whenever lone pairs are present, the molecular geometry is a subset of the electron-domain geometry. CH₄ is tetrahedral both ways; NH₃ has tetrahedral electron-domain geometry but trigonal pyramidal molecular geometry.
"A double bond counts as two electron domains." A double bond (or triple bond) counts as a single electron domain for VSEPR purposes. CO₂ has two electron domains (two double bonds), not four.
"All molecules with four atoms bonded to a central atom are tetrahedral." Only if there are no lone pairs on the central atom. SF₄ has four bonds and one lone pair, giving a seesaw shape rather than tetrahedral.
"Bond angles are always exactly the ideal value." Lone pairs compress bond angles below the ideal. The 109.5° tetrahedral angle drops to roughly 107° in NH₃ and roughly 104.5° in H₂O.
⚠️ Be able to go from a molecular formula to the correct molecular geometry in a few steps: Lewis structure, count domains, identify lone pairs, name the shape.
⚠️ Memorise the geometry names and their ideal bond angles for 2 through 6 electron domains. The table in Core Content above is your reference.
⚠️ Know that lone pairs on the central atom change the molecular geometry from the electron-domain geometry. You need to name both if the exam asks.
⚠️ Expanded-octet molecules (PCl₅, SF₄, ClF₃, XeF₂) are commonly tested. Remember that lone pairs in a trigonal bipyramidal arrangement sit equatorial.
⚠️ Bond angles: expect questions asking whether angles are exactly 109.5° or "less than 109.5°" depending on lone pairs.
True or false: A triple bond counts as three electron domains in VSEPR theory.
Fill in the blank: NH₃ has ___ electron domains and ___ lone pair(s) on nitrogen, giving a ___ molecular shape.
True or false: The molecular geometry of H₂O is linear because there are only two atoms bonded to oxygen.
Fill in the blank: In a trigonal bipyramidal arrangement, lone pairs preferentially occupy ___ positions.
True or false: BeCl₂ and CO₂ both have a linear molecular geometry.
(Answers: 1. False, it counts as one. 2. Four domains, one lone pair, trigonal pyramidal. 3. False, H₂O is bent because of two lone pairs. 4. Equatorial. 5. True.)
Q: What is the molecular geometry of BeCl₂? What is unusual about beryllium's electron count?
A: BeCl₂ is linear (AX₂, 180°). Beryllium has only 2 valence electrons and forms only 2 bonds, so it does not satisfy the octet rule. It is an electron-deficient molecule.
Q: Predict the shape of BF₃ and its bond angles.
A: BF₃ has 3 electron domains, all bonds, and no lone pairs (AX₃). The shape is trigonal planar with 120° bond angles. Boron, like beryllium, is electron-deficient (only 6 electrons around it).
Q: NH₃ and NH₄⁺ both have nitrogen as the central atom. Why do they have different molecular shapes?
A: NH₃ has 4 electron domains (3 bonds + 1 lone pair), making it trigonal pyramidal (AX₃E₁). NH₄⁺ has 4 electron domains (4 bonds, no lone pairs) because nitrogen has donated its lone pair to bond with the extra H⁺. The shape is tetrahedral (AX₄, 109.5°).
Q: Predict the molecular geometry and approximate bond angle of SF₄.
A: Sulfur has 6 valence electrons. With 4 F atoms bonded and 1 lone pair, there are 5 electron domains (trigonal bipyramidal electron-domain geometry). The lone pair sits equatorial. Molecular shape: seesaw (AX₄E₁). Bond angles are less than 90° (axial-equatorial) and less than 120° (equatorial-equatorial).
Q: XeF₂ has 5 electron domains around xenon. How many are lone pairs, and what is the molecular shape?
A: Xenon has 8 valence electrons. Two are used for 2 Xe-F bonds (2 domains), leaving 6 electrons as 3 lone pairs. Total: 5 domains (AX₂E₃). The three lone pairs occupy equatorial positions, and the two F atoms sit axial. Molecular shape: linear, 180°.
Q: Compare the CO₂ and NOCl molecules. What are their similarities and differences?
A: Both have three atoms. CO₂ has 2 double bonds, no lone pairs on C, and is linear (AX₂, 180°). NOCl has 1 double bond (N=O), 1 single bond (N-Cl), and 1 lone pair on N. With 3 electron domains, the electron-domain geometry is trigonal planar, but the molecular shape is bent (AX₂E₁), with a bond angle less than 120°.
Molecular geometry determines whether a molecule is polar overall (a polar molecule has a net dipole moment), which feeds into intermolecular forces, boiling points, and solubility. CO₂ is linear and non-polar despite having polar bonds; H₂O is bent and polar. This distinction matters heavily in later topics on solutions and phase behaviour. The expanded-octet molecules (PCl₅, SF₄, etc.) connect to the concept of d-orbital participation in bonding, which you will revisit in inorganic chemistry.
VSEPR, valence shell electron pair repulsion, electron domain, electron group, molecular geometry, molecular shape, bond angle, AXE notation, linear, trigonal planar, bent, V-shaped, tetrahedral, trigonal pyramidal, trigonal bipyramidal, seesaw, sawhorse, T-shaped, octahedral, square pyramidal, square planar, expanded octet, lone pair repulsion, general chemistry, molecular modelling lab