Simple molecular structures
3 marks · NormalExplain why bromine (Br2) is a liquid at room temperature, whereas chlorine (Cl2) is a gas, in terms of molecular mass and intermolecular forces.
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Must show: bromine has higher molecular mass than chlorine, higher mass leads to stronger intermolecular forces, stronger intermolecular forces keep Br2 as liquid at room temperature, Cl2 as gas
Also accept: More energy needed to separate Br2 molecules; weaker forces in Cl2 allow it to remain as a gas
Common mistakes: confusing covalent bond with intermolecular forces; stating only melting/boiling point without explanation
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Marking points
B1 · 1 mark · Bromine has a higher molecular mass than chlorine.
B2 · 1 mark · This leads to stronger intermolecular forces between Br2 molecules compared to Cl2.
B3 · 1 mark · As a result, bromine remains liquid while chlorine is gaseous at room temperature.
Worked solution
Bromine (Br2) has a higher molecular mass (Mr = 160) than chlorine (Cl2, Mr = 71).
A higher molecular mass means there are stronger intermolecular forces between bromine molecules than between chlorine molecules.
Therefore, more energy is needed to separate bromine molecules, so Br2 is a liquid at room temperature, while Cl2 remains a gas.