AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Pairs of electrons in the outer shell of atoms arrange themselves as far…

    Full specification wording

    Shapes of simple molecules and ions: 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.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 2 of 5
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    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.

    What earns the marks

    • Stating that the electron pairs move as far apart as possible.
    • The reason, to minimise repulsion between the pairs.
    • The correct arrangement for the total number of electron pairs.
    • The bond angle quoted for that arrangement.

    Common mistakes and how to put them right

    • 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.

    Exam tips

    • 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.

    Try the lesson

    The start of the Shapes of simple molecules and ions: Pairs of electrons in the outer shell of atoms arrange themselves as far apart as possible to minimise repulsion. lesson, made from checked teaching notes for this specification point.

    Have a go first

    In a molecule of methane, CH₄, the carbon atom has four bonding pairs of electrons. According to electron pair repulsion theory, how do these four pairs arrange themselves?

    Listen

    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. A strong answer earns marks for these points. stating that the electron pairs move as far apart as possible. the reason, to minimise repulsion between the pairs. the correct arrangement for the total number of electron pairs. the bond angle quoted for that arrangement. Exam tip: Learn the five arrangements and their angles as a single list before attempting any shape question.

    See it

    A top-down flowchart. The top node, 'Electron pairs in outer shell', leads to 'Arrange as far apart as possible'. That node leads to two nodes: 'Minimise repulsion between pairs' and 'Number of pairs fixes arrangement'. The latter branches into six nodes: 'Two pairs: linear, 180°'; 'Three pairs: trigonal planar, 120°'; 'Four pairs: tetrahedral, 109.5°'; 'Five pairs: trigonal bipyramid, 120° in plane and 90° to axial'; 'Six pairs: octahedron, all angles 90°'; and 'Named shape is arrangement of atoms only'. The last of these leads to a final node, 'Differs when lone pairs are present'.
    It shows that electron pairs arrange as far apart as possible to minimise repulsion, and that the number of pairs fixes the arrangement and its bond angles, while the named shape depends on the atoms only.

    Key words

    Say what each word means before you turn the card.

    maximum separation

    Try answering aloud or in your head before turning the card.

    minimising repulsion

    Try answering aloud or in your head before turning the card.

    linear and trigonal planar

    Try answering aloud or in your head before turning the card.

    tetrahedral arrangement

    Try answering aloud or in your head before turning the card.

    See every definition
    maximum separation
    Pairs of electrons in the outer shell of atoms arrange themselves as far apart as possible.
    minimising repulsion
    Electron pairs take up the arrangement that puts them as far apart as possible because that minimises the repulsion between them.
    linear and trigonal planar
    Two pairs go to opposite sides, linear at 180°; three spread into a flat triangle, trigonal planar at 120°.
    tetrahedral arrangement
    Four pairs move into three dimensions, tetrahedral at 109.5°.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The top node, 'Electron pairs in outer shell', leads to 'Arrange as far apart as possible'. That node leads to two nodes: 'Minimise repulsion between pairs' and 'Number of pairs fixes arrangement'. The latter branches into six nodes: 'Two pairs: linear, 180°'; 'Three pairs: trigonal planar, 120°'; 'Four pairs: tetrahedral, 109.5°'; 'Five pairs: trigonal bipyramid, 120° in plane and 90° to axial'; 'Six pairs: octahedron, all angles 90°'; and 'Named shape is arrangement of atoms only'. The last of these leads to a final node, 'Differs when lone pairs are present'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

    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.

    Carry on with the full lesson

    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: Pairs of electrons in the outer shell of atoms arrange themselves as far apart as possible to minimise repulsion.

    Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.

    Take topic quiz

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Start free

    Cookies on MasteryMind

    Essential cookies keep you signed in.

    Analytics and advertising cookies stay off unless you accept.

    Read our Privacy Policy.