2.3 Covalent bonding
6 exam questions worth 10 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
0 / 10
How many covalent bonds are there in a molecule of ammonia, NH3?
Choose one answer.
Correct Not quite: the answer is 3
Mark scheme
C
Model answer
C: nitrogen shares one pair with each of the three hydrogen atoms.
What is a covalent bond?
Mark scheme
- a shared pair of electrons (1)
- between two (non-metal) atoms (1)
Model answer
A covalent bond is a pair of electrons shared between two non-metal atoms.
Describe the bonding in a nitrogen molecule, N2.
Mark scheme
- a triple bond / three shared pairs of electrons (1)
- so each nitrogen atom has a full outer shell of 8 (1)
Model answer
The two nitrogen atoms share three pairs of electrons (a triple bond), so each has a full outer shell of 8 electrons.
Explain why a hydrogen atom forms one covalent bond but an oxygen atom forms two.
Mark scheme
- hydrogen has 1 electron and needs 1 more to fill its shell (of 2) (1)
- oxygen has 6 outer electrons and needs 2 more to get 8 (1)
Model answer
Hydrogen needs one more electron to fill its first shell, so it shares one pair. Oxygen has 6 outer electrons and needs 2 more, so it shares two pairs.
Give one advantage and one limitation of using a dot and cross diagram to represent a molecule.
Mark scheme
- advantage: shows which atom the electrons came from / shows the shared pairs clearly (1)
- limitation: does not show the 3D shape / the relative sizes of the atoms / electrons are actually identical (1)
Model answer
It clearly shows which atom each shared electron came from, but it does not show the 3D shape of the molecule.
How many electrons are shared in total in a molecule of carbon dioxide, O=C=O?
Mark scheme
- 8 (1)
Model answer
There are two double bonds; each is two shared pairs, so 4 pairs or 8 electrons.
Found a mistake?