Covalent Bonding
5 marks · EasyDefine a covalent bond.
Explain, in terms of electrostatic attraction, how a covalent bond holds two atoms together.
State one reason why many substances with simple molecular structures have low melting points.
Methane, CH, is a gas at room temperature. Explain why, using your knowledge of bonding and structure.
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(a) A covalent bond is formed when two atoms share a pair of electrons.
(b) The shared pair of electrons is attracted to both nuclei; this strong electrostatic attraction holds the atoms together in a covalent bond.
(c) The intermolecular forces between the molecules are weak.
(d) Methane has a simple molecular structure with weak intermolecular forces, so little energy is needed to separate the molecules. Therefore, it is a gas at room temperature.
Common mistakes: Saying covalent bonds involve transfer of electrons; Mixing up intramolecular and intermolecular forces
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Marking points
B1 (a) · 1 mark · Correctly defines covalent bond as sharing a pair of electrons between atoms.
M1 (b) · 1 mark · Mentions shared pair of electrons is attracted to both nuclei.
A1 (b) · 1 mark · Explains that this electrostatic attraction holds atoms together.
B2 (c) · 1 mark · Identifies weak intermolecular forces as the cause of low melting points.
B3 (d) · 1 mark · Relates methane's low melting/boiling point to simple molecular structure and weak forces between molecules.
Worked solution
a) A covalent bond is formed when two atoms share a pair of electrons.
b) A covalent bond forms because the shared pair of electrons is attracted to both nuclei of the bonded atoms. This creates a strong electrostatic attraction that holds the two atoms together.
c) Substances with simple molecular structures have low melting points because there are only weak intermolecular forces between the molecules.
d) Methane is a gas at room temperature because it consists of simple molecules with weak forces between them. These forces are easily overcome, so methane has a low boiling point and is a gas under standard conditions.