Covalent Bonding

6 marks · Normal

This question is about the structures of covalent molecules and giant covalent substances.

(a)

Draw a dot-and-cross diagram (showing only outer shell electrons) for a molecule of water (H2O).

[2 marks]

(b)

Diamond and graphite are giant covalent structures made only of carbon atoms.

i. State how many other carbon atoms each carbon is bonded to in:

• diamond

• graphite

[2 marks]

(c)

Explain, in terms of structure and bonding, why diamond has a much higher melting point than water.

[2 marks]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

(a) O in centre with two pairs of shared electrons to Hs; two lone pairs on O. · must show correct shared pairs, two lone pairs on O

(b) Diamond: 4; Graphite: 3. · must show diamond: 4, graphite: 3

(c) Diamond is a giant covalent structure where each carbon is bonded to four others with strong covalent bonds that must be broken to melt it, requiring much more energy. Water is a simple molecular substance where only weak intermolecular forces must be overcome to melt or boil. · must show diamond: giant covalent, water: simple molecular, strong covalent bonds (diamond), intermolecular forces (water)

Show mark scheme and worked solution

Marking points

D1 (a) · 1 mark · Correct sharing of electrons in H2O (O with two shared pairs to Hs).

D2 (a) · 1 mark · Shows two lone pairs on the O atom.

M1 (b) · 1 mark · States 4 for diamond.

M2 (b) · 1 mark · States 3 for graphite.

E1 (c) · 1 mark · Links high melting point in diamond to giant covalent structure and strong bonds between atoms.

E2 (c) · 1 mark · Describes that water has a simple molecular structure with weak intermolecular forces.

Worked solution

a) (Student's drawing; correct: O in centre with two pairs of shared electrons to each H, plus two lone pairs on O.)

b) Diamond: 4; Graphite: 3.

c) Diamond is a giant covalent structure made of carbon atoms with strong covalent bonds throughout. A lot of energy is needed to break all these bonds. Water is a simple molecular substance, so only weak intermolecular forces have to be broken; covalent bonds do not break when it melts. Therefore, diamond has a much higher melting point.