Molecular Shapes
VSEPRs
Valence Shell Electron Pair Repulsion
- Pairs of electrons around an atom repel each other, determining a molecules shape
- Pairs of electrons are known as ‘charge centres’ and include both the electron in a covalent
bond (a double/triple bond counts as one charge centre) and lone pairs/non-bonding pairs
- 2 basic rules:
1. All σ -bonded electron pairs & all lone pairs arrange themselves as far apart in space as
is possible, π-bonded electron pairs are excluded
2. Lone pairs repel more strongly than bonding pairs
- E.g. Ethene
The carbon has 2 charge centres (C–H and C=C)
The push as far away from each other as possible making a 180º bond angle
Drawing shapes in 3D
1. Draw as many atoms as you can in the same plane ‘flat’ on the paper
2. Use solid wedges to show atoms coming out of the plane of paper towards you
3. Use dashed wedge to show the atoms going back into the plane of the paper away from you
E.g. HCl3
Working out the shape of a molecule or ion
(all rules are in reference to the central atom)
1. Count the total number of bonds (double and triple bonds count as singles for this) + lone pairs
to work out the shape.
2. Separate the number of bonding pair of electrons and lone pair of electrons
3. Refer to the chart for the shape and bond angles of the molecules.
VSEPRs
Valence Shell Electron Pair Repulsion
- Pairs of electrons around an atom repel each other, determining a molecules shape
- Pairs of electrons are known as ‘charge centres’ and include both the electron in a covalent
bond (a double/triple bond counts as one charge centre) and lone pairs/non-bonding pairs
- 2 basic rules:
1. All σ -bonded electron pairs & all lone pairs arrange themselves as far apart in space as
is possible, π-bonded electron pairs are excluded
2. Lone pairs repel more strongly than bonding pairs
- E.g. Ethene
The carbon has 2 charge centres (C–H and C=C)
The push as far away from each other as possible making a 180º bond angle
Drawing shapes in 3D
1. Draw as many atoms as you can in the same plane ‘flat’ on the paper
2. Use solid wedges to show atoms coming out of the plane of paper towards you
3. Use dashed wedge to show the atoms going back into the plane of the paper away from you
E.g. HCl3
Working out the shape of a molecule or ion
(all rules are in reference to the central atom)
1. Count the total number of bonds (double and triple bonds count as singles for this) + lone pairs
to work out the shape.
2. Separate the number of bonding pair of electrons and lone pair of electrons
3. Refer to the chart for the shape and bond angles of the molecules.
