Part 2: Electron-pair repulsion and molecular shapes

Count bonding regions and lone pairs around the selected atom, then explain the shape and bond angles.

Count regions around the central atom

Electron pairs repel and arrange to minimise repulsion. Count a multiple bond as one bonding region for the basic geometry; include central lone pairs. The molecular shape names atom positions, so it omits the invisible lone pairs. Lone-pair–lone-pair repulsion is strongest, followed by lone-pair–bonding-pair, then bonding-pair–bonding-pair.

For NH₃, nitrogen has three bonding pairs and one lone pair: four regions with tetrahedral electron arrangement, but a pyramidal molecular shape. The lone pair compresses H–N–H to about 107°. In water two lone pairs compress the H–O–H angle to 104.5°. These are specific examples, not a universal 2.5° subtraction rule.

Count electrons before naming an unfamiliar shape

For a simple central atom with single bonds to monovalent surrounding atoms, count the central valence electrons plus one from each surrounding atom, add electrons for negative charge or subtract for positive charge, then divide by two. This gives the number of electron pairs around the centre in that limited model. It is not a universal shortcut for structures containing multiple bonds.

Example PCl₃: 5 + 3 = 8 electrons, or four pairs. Three are bonding pairs and one is a lone pair; molecular shape is pyramidal. Example NH₄⁺: 5 + 4 − 1 = 8, again four pairs, but all four are bonding, so the shape is tetrahedral.

For CO₂, first draw O=C=O. The central carbon has two regions of electron density and no lone pairs. Four shared pairs are present in the two double bonds, but those pairs occupy two bonding directions. Treating them as four independently directed single bonds would incorrectly predict tetrahedral geometry.

Core shapes up to six electron-pair regions

Write a shape answer as a cause-and-effect chain

For water, say that oxygen has two bonding pairs and two lone pairs. Four regions arrange approximately tetrahedrally to minimise repulsion. Lone pairs repel more strongly than bonding pairs and compress the H–O–H angle; the molecule is bent with angle about 104.5°. “It is bent because it has lone pairs” omits the electron-pair arrangement and repulsion.

For an unfamiliar species do not force the water angle onto every bent molecule. A centre with two bonding regions and one lone pair begins from a three-region arrangement; its angle is below the corresponding 120° ideal, rather than necessarily 104.5°. Bond types and ligands also affect actual angles.

Names refer to atom positions, while electron-region geometry includes lone pairs. Four regions can therefore produce tetrahedral, pyramidal or bent molecular shapes. Drawing the lone pairs explicitly prevents confusing those two descriptions.

Apply the same counting to unfamiliar species

Five regions arrange trigonal bipyramidally; a lone pair favours an equatorial position. Four bonds and one lone pair give a seesaw shape, three bonds and two lone pairs a T shape, and two bonds and three lone pairs a linear molecule. Six regions are octahedral; four bonds and two opposite lone pairs give square planar geometry. Use information provided to count the electrons rather than memorising an isolated list.

Represent three-dimensional tetrahedral geometry with two bonds in the page, a solid wedge towards the viewer and a dashed wedge away. A flat cross does not show the tetrahedron. For CO₂, two C=O bonds give two regions around carbon and no lone pairs, so it is linear despite having four shared electron pairs in total.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources