Shapes of Molecules and Ions

Shapes of Molecules

Define electron pair repulsion (EPR) theory.

Electron pairs around a central atom arrange themselves to minimise repulsion between them.

What is another name for electron pair repulsion theory?

Valence shell electron pair repulsion theory (VSEPR).

What determines the shape of a molecule or ion?

The arrangement of bonding pairs and lone pairs of electrons around the central atom.

Why do electron pairs spread out around a central atom?

Electron pairs repel each other and arrange themselves as far apart as possible to minimise repulsion.

What is the order of electron-pair repulsion strength?

Lone pair–lone pair > lone pair–bond pair > bond pair–bond pair.

Which type of electron-pair repulsion is strongest?

Lone pair–lone pair repulsion.

Which type of electron-pair repulsion is weakest?

Bond pair–bond pair repulsion.

Why do lone pairs repel more strongly than bonding pairs?

Lone pairs have greater electron density around the central atom and occupy more space.

Define a bond angle.

The angle between two bonds from the same central atom.

How do you predict the shape of a molecule or ion?

Determine the number of bonding pairs and lone pairs around the central atom, then arrange them to minimise electron-pair repulsion.

How should a double or triple bond be treated when predicting molecular shape?

As one region of electron density, equivalent to one bonding pair for determining shape.

What shape results from 2 bonding pairs and 0 lone pairs?

Linear.

What is the bond angle in a linear molecule?

180°.

Give an example of a linear molecule.

BeCl₂.

What is the shape and bond angle of BeCl₂?

Linear, 180°.

What is the shape and bond angle of CO₂?

Linear, 180°.

Why is CO₂ linear?

The carbon has two regions of electron density and no lone pairs, so they arrange 180° apart.

What shape results from 3 bonding pairs and 0 lone pairs?

Trigonal planar.

What is the bond angle in a trigonal planar molecule?

120°.

Give an example of a trigonal planar molecule.

BCl₃.

What is the shape and bond angle of BCl₃?

Trigonal planar, 120°.

What shape results from 4 bonding pairs and 0 lone pairs?

Tetrahedral.

What is the bond angle in a tetrahedral molecule?

109.5°.

Give an example of a tetrahedral molecule.

CH₄.

What is the shape and bond angle of CH₄?

Tetrahedral, 109.5°.

Why is CH₄ tetrahedral?

Four bonding pairs repel equally and arrange themselves as far apart as possible.

What shape results from 5 bonding pairs and 0 lone pairs?

Trigonal bipyramidal.

What bond angles occur in a trigonal bipyramidal molecule?

90° and 120°.

Give an example of a trigonal bipyramidal molecule.

PCl₅.

What is the shape and bond angles of PCl₅?

Trigonal bipyramidal, 90° and 120°.

What shape results from 6 bonding pairs and 0 lone pairs?

Octahedral.

What bond angles occur in an octahedral molecule?

90° and 180°.

Give an example of an octahedral molecule.

SF₆.

What is the shape and bond angles of SF₆?

Octahedral, 90° and 180°.

Why is there a 180° bond angle in SF₆?

Two fluorine atoms can be positioned directly opposite each other.

What shape results from 3 bonding pairs and 1 lone pair?

Trigonal pyramidal.

Give an example of a trigonal pyramidal molecule.

NH₃.

What is the shape and bond angle of NH₃?

Trigonal pyramidal, 107°.

Why is the bond angle in NH₃ 107° rather than 109.5°?

Lone pair–bond pair repulsion is stronger than bond pair–bond pair repulsion, pushing the N–H bonds closer together.

How does a lone pair affect bond angles?

It repels bonding pairs more strongly and therefore decreases the bond angle.

What shape results from 2 bonding pairs and 2 lone pairs?

V-shaped.

Give an example of a V-shaped molecule.

H₂O.

What is the shape and bond angle of H₂O?

V-shaped, 104.5°.

Why is the bond angle in H₂O only 104.5°?

Two lone pairs repel the bonding pairs strongly, pushing the O–H bonds closer together.

Why is the H₂O bond angle smaller than the NH₃ bond angle?

H₂O has two lone pairs whereas NH₃ has one, so there is greater lone-pair repulsion in H₂O.

What happens to the tetrahedral bond angle when one lone pair is present?

It decreases from 109.5° to about 107°.

What happens to the tetrahedral bond angle when two lone pairs are present?

It decreases further to about 104.5°.

What is the shape and bond angle of NH₄⁺?

Tetrahedral, 109.5°.

Why does NH₄⁺ have a larger bond angle than NH₃?

NH₄⁺ has four bonding pairs and no lone pairs, whereas NH₃ has one lone pair that causes stronger repulsion.

Why is NH₄⁺ tetrahedral?

It has four bonding pairs and no lone pairs around the nitrogen atom.

What is the shape around each carbon atom in ethene, C₂H₄?

Trigonal planar.

What are the bond angles around each carbon atom in ethene?

Approximately 120°.

Why is ethene planar?

Each carbon has three regions of electron density and no lone pairs, giving trigonal planar geometry.

How is the C=C double bond treated when predicting the shape of ethene?

As one region of electron density.

State the shape and bond angle for 2 bonding pairs and 0 lone pairs.

Linear, 180°.

State the shape and bond angle for 3 bonding pairs and 0 lone pairs.

Trigonal planar, 120°.

State the shape and bond angle for 4 bonding pairs and 0 lone pairs.

Tetrahedral, 109.5°.

State the shape and bond angles for 5 bonding pairs and 0 lone pairs.

Trigonal bipyramidal, 90° and 120°.

State the shape and bond angles for 6 bonding pairs and 0 lone pairs.

Octahedral, 90° and 180°.

State the shape and bond angle for 3 bonding pairs and 1 lone pair.

Trigonal pyramidal, about 107°.

State the shape and bond angle for 2 bonding pairs and 2 lone pairs.

V-shaped, about 104.5°.

What three steps should you use in an exam to explain molecular shape?

Count the bonding and lone pairs, state that electron pairs repel and arrange to minimise repulsion, then use the relative strength of repulsions to explain the bond angle.