Shapes of simple molecules and ions — AQA A-Level Chemistry
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Shapes of simple molecules and ions explained
Electron pair repulsion theory treats each pair of outer-shell electrons around the central atom as a cloud of negative charge.
Read the full explanation
Like charges repel, so the clouds push each other as far apart as they can, and the positions they settle into fix the shape. Both kinds of pair count: bonding pairs, shared with another atom, and lone pairs, which belong to the central atom alone. A lone pair is attracted by only one nucleus rather than two, so it stays closer to the central atom and spreads out more, occupying more space and repelling its neighbours harder. That is why lone pairs must be counted when predicting a shape even though they are invisible in the finished molecule.
Pairs of electrons in the outer shell of atoms arrange themselves as far apart as possible to minimise repulsion.
The governing rule is that electron pairs take up the arrangement that puts them as far apart as possible, because that minimises the repulsion between them. The number of pairs fixes the arrangement: two pairs go to opposite sides, linear at 180°; three spread into a flat triangle, trigonal planar at 120°; four move into three dimensions, tetrahedral at 109.5°; five form a trigonal bipyramid with angles of 120° in the plane and 90° to the axial positions; six form an octahedron with all angles 90°. These are the arrangements of the electron pairs. The shape you name is the arrangement of the atoms only, which differs whenever lone pairs are present.
Lone pair–lone pair repulsion is greater than lone pair– bond pair repulsion, which is greater than bond pair–bond pair repulsion.
Not all repulsions are equal. Lone pair to lone pair repulsion is the strongest, lone pair to bonding pair is intermediate, and bonding pair to bonding pair is the weakest, because a lone pair is held by one nucleus only and so spreads closer to the central atom. The consequence is that each lone pair squeezes the remaining bond angles by roughly 2.5°. Methane, with four bonding pairs, keeps the full 109.5°. Ammonia, with three bonding pairs and one lone pair, closes to 107°. Water, with two bonding pairs and two lone pairs, closes further to 104.5°. The same reasoning gives about 104° for a four-pair ion carrying two lone pairs, such as ClF₂⁺.
The effect of electron pair repulsion on bond angles.
Bond angle follows from two numbers: the total electron pairs, which sets the base arrangement, and how many of those are lone pairs. With no lone pairs the base angle stands: methane and the ammonium ion are 109.5°, boron trifluoride 120°, carbon dioxide 180°, sulfur hexafluoride 90°. For a tetrahedral base, each lone pair closes the angle by about 2.5°: ammonia is 107°, water 104.5°. This 2.5° rule does not apply to all geometries; for example, square planar shapes maintain 90° angles as lone pair repulsions cancel. A trigonal bipyramid has two different angles, 120° equatorial and 90° axial. Quote an angle consistent with the shape you drew to avoid contradicting yourself.
Students should be able to: explain the shapes of, and bond angles in, simple molecules and ions with up to six electron pairs (including lone pairs of electrons) surrounding the central atom.
Work to a fixed routine. Count the outer-shell electrons of the central atom, add one for each atom singly bonded to it, then adjust for charge by subtracting the positive charge or adding the negative charge. Divide the total by 2 to give the number of electron pairs, and subtract the number of bonded atoms to give the lone pairs. The pair count fixes the arrangement, the lone pairs reduce the angles, and the shape is named from the positions of the atoms alone. So four pairs with one lone pair is trigonal pyramidal at 107°, four pairs with two lone pairs is v-shaped or bent at about 104.5°, and six pairs with none is octahedral at 90°.
Your focus
- Count the bonding pairs and lone pairs around a central atom from a dot-and-cross diagram.
- Explain why a lone pair occupies more space around the central atom than a bonding pair.
- Justify why lone pairs must be included when predicting a molecular shape.
Show all 15 objectives
- Predict the arrangement adopted by two, three, four, five or six electron pairs around a central atom.
- State the bond angle associated with each of those arrangements.
- Explain why the named shape of a molecule can differ from the arrangement of its electron pairs.
- State the order of the three types of electron pair repulsion and use it to explain a reduced bond angle.
- Predict the bond angle in a four-pair species from the number of lone pairs it carries.
- Explain why the bond angle in water is smaller than that in ammonia.
- Predict the bond angle in a molecule or ion from its total electron pairs and its lone pair count.
- Quote both bond angles present in a trigonal bipyramidal species and say which positions they connect.
- Apply the 2.5° angle reduction rule correctly to tetrahedral base geometries without overgeneralising to other shapes.
- Calculate the number of bonding and lone pairs around the central atom of a molecular ion, adjusting for its charge.
- Draw and name the shape of a species with up to six electron pairs, including any lone pairs that influence it.
- Explain a predicted shape and bond angle using the relative strengths of electron pair repulsions.
Shapes of simple molecules and ions exam tips
Marking Points
- one mark for describing electron pairs as regions or clouds of negative charge
- one mark for stating that these regions repel one another
- one mark for including lone pairs as well as bonding pairs in the count that determines the shape
- one mark for a lone pair occupying more space because it is attracted by only one nucleus
- one mark for stating that the electron pairs move as far apart as possible
- one mark for the reason, to minimise repulsion between the pairs
- one mark for the correct arrangement for the total number of electron pairs
- one mark for the bond angle quoted for that arrangement
- one mark for the order lone pair to lone pair greater than lone pair to bonding pair greater than bonding pair to bonding pair
- one mark for a lone pair occupying more space around the central atom than a bonding pair
- one mark for the number of lone pairs on the central atom correctly identified
- one mark for a bond angle consistent with that number of lone pairs
- one mark for the total number of electron pairs around the central atom
- one mark for the number of those pairs that are lone pairs
- one mark for the base bond angle of that arrangement
- one mark for the final bond angle stated for the shape to the nearest 0.5°
- one mark for the correct numbers of bonding pairs and lone pairs, adjusted for the charge on an ion
- one mark for a three-dimensional diagram using wedges and dashed lines to show the shape
- one mark for naming the shape from the positions of the atoms
- one mark for the bond angle consistent with the lone pairs present
- one mark for an explanation in terms of repulsion between electron pairs
Examiner Tips
- 💡Count total electron pairs first, then separate them into bonding and lone pairs.
- 💡Say charge clouds repel rather than electrons repel; the pair is the unit in this theory.
- 💡When a question says include any lone pairs that influence the shape, they must appear in your drawing.
- 💡Learn the five arrangements and their angles as a single list before attempting any shape question.
- 💡Use wedges and dashed lines so a three-dimensional shape is unambiguous on paper.
- 💡Write the number of pairs beside your diagram; it justifies the arrangement you chose.
- 💡Write the repulsion order out in full using the words greater than; a shorthand ordering can be misread.
- 💡Subtract 2.5° per lone pair from the base angle of the arrangement and state that is what you have done.
- 💡Angles within a range are usually accepted, so a reasoned value close to the accepted one still scores.
- 💡Write the pair count and lone pair count beside the diagram so the examiner can follow your reasoning.
- 💡For trigonal bipyramidal species give both angles and say which positions each refers to.
- 💡Use the 2.5° reduction rule specifically for tetrahedral base shapes (109.5° to 107° to 104.5°), but remember it does not apply universally to all geometries.
- 💡Show the electron count as working; it earns credit even if the final shape is misnamed.
- 💡Use wedges and dashes on every three-dimensional shape and keep bond lengths roughly equal.
- 💡Answer all three parts when asked: draw, name and give the angle, plus the repulsion explanation.
Common Mistakes
- counting only the atoms attached and ignoring the lone pairs
- saying lone pairs do not affect the shape because they cannot be seen
- treating individual electrons rather than pairs as the repelling units
- assuming a bonding pair repels more strongly than a lone pair
- quoting 109.5° for any molecule with four electron pairs, including those with lone pairs
- drawing a tetrahedral molecule flat on the page with 90° angles
- saying the pairs repel in order to become stable rather than to minimise repulsion
- naming the shape from the electron pair arrangement rather than from the positions of the atoms
- reversing the order so bonding pairs are said to repel most strongly
- saying lone pairs push bonding pairs further apart, widening the angle
- swapping the angles, giving water 107° and ammonia 104.5°
- reducing the angle by 2.5° once regardless of how many lone pairs are present
- forgetting to adjust the electron count for the charge on an ion, so the lone pair count is wrong
- quoting the unchanged base angle when lone pairs are present
- applying the 2.5° reduction rule to non-tetrahedral geometries like square planar or octahedral
- quoting an angle that contradicts the shape drawn alongside it
- leaving the charge on an ion out of the electron count, giving the wrong number of lone pairs
- naming a shape from all the electron pairs, so calling water tetrahedral rather than v-shaped
- drawing the shape flat so that it is ambiguous
- omitting the lone pairs from the diagram when the question asks for those that influence the shape