Shapes of Molecules

Question 1

Molecules and molecular ions come in loads of different shapes. What does this shape depend on?

Answer 1

The number of pairs of electrons in the outer shell of the central atom.

Question 2

What are shared electrons called?

Answer 2

Bonding pairs.

Question 3

What are unshared electrons called?

Answer 3

Lone pairs or non-bonding pairs.

Question 4

What electron pair repels its other electron pairs more, lone or bonding pairs?

Answer 4

Lone pairs repel more than bonding pairs. So, the greatest angles are between lone pairs of electrons, and bond angles between bonding pairs are often reduced because they are pushed together by lone pair repulsion.
This is known as Valence-Shell Electron-Pair Repulsion Theory.

Question 5

What do molecules with two electron pairs have a bond angle of and why is this?

Answer 5

180 degrees and have a linear shape. This is because the pairs of bonding electrons want to be as far away from each other as possible.

Question 6

What are the bond angles if there are three bonding pairs of electrons?

Answer 6

The repulsion of the charge clouds is the same between each pair and so the bond angles are all 120 degrees.
The shape of the molecule is trigonal planar.

Question 7

What shape and bond angle will you get if there are two bonding pairs of electrons and one lone pair in a molecule?

Answer 7

You’ll get a squished trigonal planar shape which is called non-linear. The bond angle will be a bit less than 120 degrees.

Question 8

What is the shape and bond angle if there are three bonding pairs of electrons and one lone pair?

Answer 8

The bond angle is 107 degrees and the shape of the molecule is trigonal pyramidal.

Question 9

What will happen if there are two bonding pairs of electrons and two lone pairs and what is the shape and bond angle?

Answer 9

The lone pair/lone pair repulsion will squish the bond angle even further.
The bond angle is 104.5 degrees and the shape of the molecules is non linear.

Question 10

What shape will a molecule with five bonding pairs be?

Answer 10

Trigonal bipyramidal.

Question 11

What does the repulsion between the bonding pairs in a trigonal bipyramidal mean?

Answer 11

It means that three of the atoms will form a trigonal planar shape with bond angles of 120 degrees and the other two atoms will be at 90 degrees to them.

Question 12

What shape will a molecule with six bonding pairs be?

Answer 12

Octahedral and all of the bond angles are 90 degrees.

Question 13

Give an example of molecules with two electron pairs.

Answer 13

Beryllium chloride

Carbon dioxide

Question 14

Give an example of molecules with three electron pairs.

Answer 14

Aluminium chloride

Question 15

Give an example of molecules with four electron pairs.

Answer 15

Methane

Ammonium ion

Question 16

Give an example of molecules with five electron pairs.

Answer 16

Phosphorus pentachloride

Question 17

Give an example of molecules with six electron pairs.

Answer 17

Sulfur hexaflouride

Question 18

A molecule has 2 bonding pairs what is its shape and bond angle?

Answer 18

Linear

180 degrees

Question 19

A molecule has 3 bonding pairs what is its shape and bond angle?

Answer 19

Trigonal planar

120 degrees

Question 20

A molecule has 4 bonding pairs what is its shape and bond angle?

Answer 20

Tetrahedral

109.5 degrees

Question 21

A molecule has 5 bonding pairs what is its shape and bond angle?

Answer 21

Trigonal bipyramidal

90 degrees and 120 degrees

Question 22

A molecule has 6 bonding pairs what is its shape and bond angle?

Answer 22

Octahedral

90 degrees

Question 23

Chemists are able to predict the shape of a simple covalent molecule from the number of electron pairs surrounding the central atom.
Explain how this enables chemists to predict the shape.

Answer 23

Pairs of (electrons surrounding a central atom) repel.
The shape is determined by the number of bond pairs AND
the number of lone pairs (of electrons).

Question 24

Molecules of BF3 contain polar bonds, but the molecules are non-polar. Suggest an explanation for this difference.

Answer 24

BF3 is symmetrical

The dipoles cancel out

Question 25

Describe how van der Waals’ forces arise.

Answer 25

Dipole caused by uneven distribution of electrons.
Creates a temporary dipole.
This causes induced dipoles in neighbouring
molecules.