AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: explain the shapes of, and bond angles in,…
Full specification wording
Shapes of simple molecules and ions: 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.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 5 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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°.
What earns the marks
- The correct numbers of bonding pairs and lone pairs, adjusted for the charge on an ion.
- A three-dimensional diagram using wedges and dashed lines to show the shape.
- Naming the shape from the positions of the atoms.
- The bond angle consistent with the lone pairs present.
- An explanation in terms of repulsion between electron pairs.
Common mistakes and how to put them right
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.
Exam tips
- 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.
Try the lesson
The start of the Shapes of simple molecules and ions: 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. lesson, made from checked teaching notes for this specification point.
Have a go first
What is the shape of a methane molecule, CH₄, which has four bonding pairs and no lone pairs around the central carbon atom?
Listen
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°. A strong answer earns marks for these points. the correct numbers of bonding pairs and lone pairs, adjusted for the charge on an ion. a three-dimensional diagram using wedges and dashed lines to show the shape. naming the shape from the positions of the atoms. the bond angle consistent with the lone pairs present. Exam tip: Show the electron count as working; it earns credit even if the final shape is misnamed.
Key words
Say what each word means before you turn the card.
adjusting for charge
Try answering aloud or in your head before turning the card.
naming shapes from atoms
Try answering aloud or in your head before turning the card.
repulsion explanation
Try answering aloud or in your head before turning the card.
See every definition
- adjusting for charge
- Subtract the positive charge or add the negative charge when counting the outer-shell electrons of the central atom.
- naming shapes from atoms
- The shape is named from the positions of the atoms alone, not from all the electron pairs.
- repulsion explanation
- An explanation in terms of repulsion between electron pairs, where the lone pairs reduce the angles.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Shapes of simple molecules and ions: 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.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge