Shapes of molecules

· VSEPR theory = Valence Shell Electron Pair Repulsion theory.
· Used to state, explain and predict molecular shapes and bond angles.
· Main idea: electron pairs around a central atom repel each other and arrange themselves as far apart as possible.
· Both bonding pairs and lone pairs affect the final shape.
· The shape of a molecule is described by the arrangement of atoms, not the arrangement of lone pairs.
· Exam focus: identify the central atom, count regions of electron density, then link to the correct shape and bond angle.

This image set summarises the main VSEPR molecular geometries. Use it to compare how changing the number of bonding pairs and lone pairs changes the shape and bond angles. Source

Linear molecules

· Linear shape = atoms arranged in a straight line.
· Bond angle = 180°.
· Example: CO₂ is linear, 180°.
· Around the central carbon in CO₂, there are 2 regions of electron density and no lone pairs on the central atom.
· Multiple bonds count as one region of electron density for VSEPR shape prediction.

This diagram helps students recognise simple VSEPR shapes such as linear and trigonal planar. It is useful for comparing molecules with different numbers of bonding groups around the central atom. Source

Trigonal planar molecules

· Trigonal planar shape = 3 bonding pairs arranged in one flat plane.
· Bond angle = 120°.
· Example: BF₃ is trigonal planar, 120°.
· The central atom has 3 bonding regions and no lone pairs.
· “Planar” means the atoms lie in the same plane.

Tetrahedral molecules

· Tetrahedral shape = 4 bonding pairs arranged evenly in 3D.
· Bond angle = 109.5°.
· Example: CH₄ is tetrahedral, 109.5°.
· The central carbon has 4 bonding pairs and no lone pairs.
· This is a key reference shape for molecules with four electron pairs around the central atom.

This diagram links Lewis structures to 3D molecular shapes. It is useful for seeing how electron-pair repulsion produces common shapes such as tetrahedral, pyramidal and non-linear molecules. Source

Molecules with lone pairs: pyramidal and non-linear

· Lone pairs repel more strongly than bonding pairs, so they can reduce bond angles.
· NH₃ has 3 bonding pairs + 1 lone pair around nitrogen.
· Shape of NH₃ = pyramidal, bond angle = 107°.
· The bond angle in NH₃ is less than 109.5° because the lone pair compresses the N–H bonds.
· H₂O has 2 bonding pairs + 2 lone pairs around oxygen.
· Shape of H₂O = non-linear / bent, bond angle = 104.5°.
· H₂O has a smaller bond angle than NH₃ because it has two lone pairs, causing greater repulsion.

Expanded shapes: trigonal bipyramidal and octahedral

· Some molecules have 5 or 6 bonding pairs around the central atom.
· PF₅ has 5 bonding pairs and no lone pairs.
· Shape of PF₅ = trigonal bipyramidal.
· Bond angles in PF₅ = 120° and 90°.
· SF₆ has 6 bonding pairs and no lone pairs.
· Shape of SF₆ = octahedral.
· Bond angle in SF₆ = 90°.
· These are key examples of expanded octet shapes in Period 3 compounds.

This interactive model helps students visualise 3D molecular shapes. It is especially useful for understanding why PF₅ has both 90° and 120° angles, while SF₆ has 90° angles. Source

Core examples to memorise

· BF₃ = trigonal planar, 120°.
· CO₂ = linear, 180°.
· CH₄ = tetrahedral, 109.5°.
· NH₃ = pyramidal, 107°.
· H₂O = non-linear / bent, 104.5°.
· SF₆ = octahedral, 90°.
· PF₅ = trigonal bipyramidal, 120° and 90°.

How to predict shapes in exam questions

· Draw or imagine the dot-and-cross / Lewis structure.
· Identify the central atom.
· Count bonding pairs and lone pairs around the central atom.
· Use VSEPR: electron pairs arrange to minimise repulsion.
· State the shape name and bond angle clearly.
· If lone pairs are present, explain angle reduction using stronger lone pair–bond pair repulsion.
· For unfamiliar molecules or ions, compare them with analogous syllabus examples.

Common exam traps

· Do not call H₂O “linear”; it is non-linear / bent, 104.5°.
· Do not call NH₃ “tetrahedral”; its electron-pair arrangement is tetrahedral, but its molecular shape is pyramidal.
· Do not forget that lone pairs are not included in the shape name, but they do affect the shape and bond angle.
· Do not use 109.5° for NH₃ or H₂O; lone pairs reduce the bond angles to 107° and 104.5°.
· Do not confuse trigonal planar with trigonal bipyramidal: trigonal planar has 3 bonding pairs, trigonal bipyramidal has 5 bonding pairs.