23: Shapes of Molecules

Question 1

List 2 examples of chemicals with central atom having less than 8 electrons.

Answer 1

1. Boron trifluoride, the central boron atom only has 6 electrons.
2. Nitrogen dioxide, the central nitrogen atom only has 7 electrons.

Question 2

List 2 examples of chemicals with central atom having more than 8 electrons.

Answer 2

1. Phosphorus pentachloride, the central phosphorus atom has 10 electrons.
2. Sulphur hexafluoride, the central sulphur atom has 12 electrons.

Question 3

Why can some atoms form an expanded octet in their compounds?

Answer 3

For atoms from period 3 or lower, their outermost electron shells of these atoms can hold more than 8 electrons, therefore an expanded octet structure is possible.

Question 4

List one similarity and one difference between the structure of sulphur dioxide and ozone.

Answer 4

Similarity: the central atom is bonded to one oxygen atom with a double bond.
Difference: the sulphur atom in sulphur dioxide forms another double bond with the other oxygen atom , creating an expanded octet structure, while the central oxygen atom in ozone forms a dative covalent bond with the third oxygen atom.

Question 5

Why is an expanded octet structure usually preferred over dative covalent bonds?

Answer 5

Expanded octet structure allows the formation of more bonds, hence increasing the stability of the molecule.

Question 6

What is the shape of a boron trifluoride molecule?

Answer 6

It has a trigonal planar shape.

Question 7

What is the bond angle in a boron trifluoride molecule?

Answer 7

120º

Question 8

What is the shape of a methane molecule?

Answer 8

It has a tetrahedral shape.

Question 9

What is the bond angle in a methane molecule?

Answer 9

109.5º

Question 10

What is the shape of an ammonia molecule?

Answer 10

It has a trigonal pyramidal shape.

Question 11

What is the bond angle in a methane molecule?

Answer 11

109.5º

Question 12

What is the shape of a water molecule?

Answer 12

It has a bent shape / it is v-shaped.

Question 13

What is the shape of a phosphorus pentachloride molecule?

Answer 13

It has a trigonal bipyramidal shape.

Question 14

What is the bond angle in a phosphorus pentachloride molecule?

Answer 14

90º

Question 15

What is the shape of a sulphur hexafluoride molecule?

Answer 15

It has a octahedral shape.

Question 16

What is the bond angle in a sulphur hexafluoride molecule?

Answer 16

90º

Question 17

What is the shape of a carbon dioxide molecule?

Answer 17

It has a linear shape.

Question 18

What is the bond angle in a carbon dioxide molecule?

Answer 18

180º

Question 19

Why does carbon dioxide have a linear shape?

Answer 19

The central carbon atom forms two double bonds with the oxygen atoms. These two groups of bond electrons repel each other, thus they are split 180º apart, forming the linear shape of the carbon dioxide molecule.

Question 20

Why does ammonia have a trigonal pyramidal shape?

Answer 20

The central nitrogen atom is bonded to 3 hydrogen atoms, forming three bond pairs and leaving one lone pair left. These 4 pairs of electrons repel each other and form a tetrahedral arrangement, so the 3 bond pairs form a trigonal pyramidal shape.

Question 21

Why does water have a bent shape?

Answer 21

The central oxygen atom is bonded to 2 hydrogen atoms, forming two bond pairs and leaving two lone pairs left. These 4 pairs of electrons repel each other and form a tetrahedral arrangement, so the 2 bond pairs form a bent shape.