Barium is an alkaline earth metal in Group 2. When barium reacts with water, a metal hydroxide and a gas are produced. Explain why barium reacts more vigorously with water than magnesium does, and state the name of the gas produced in this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Alkaline earth metals react with water to form metal hydroxides and hydrogen gas. The reactivity of Group 2 elements increases down the group.
Model answer (5 marks)
Barium atoms are larger than magnesium atoms because barium has more electron shells.
The outer electrons in barium are further from the nucleus than in magnesium.
There is greater shielding of the outer electrons from the nuclear charge in barium.
The attraction between the nucleus and the outer electrons is therefore weaker, so the outer electrons are lost more easily.
Consequently barium reacts more vigorously with water.
The gas produced is hydrogen.
The outer electrons in barium are further from the nucleus than in magnesium.
There is greater shielding of the outer electrons from the nuclear charge in barium.
The attraction between the nucleus and the outer electrons is therefore weaker, so the outer electrons are lost more easily.
Consequently barium reacts more vigorously with water.
The gas produced is hydrogen.
Examiner tips
- Use the word ‘because’ to link size, shielding and reactivity. Mention the weaker attraction of the outer electrons. State the gas explicitly at the end.
Common mistakes
- Writing ‘magnesium reacts more vigorously’ instead of barium. Omitting the word ‘hydrogen’ or giving the wrong gas name. Failing to explain the role of shielding and electron distance.
Mark scheme (5 marks)
- Barium atoms are larger than magnesium atoms / barium has more electron shells than magnesium
- The outer electrons in barium are further from the nucleus than in magnesium
- There is greater shielding of the outer electrons from the nuclear charge in barium than in magnesium
- The outer electrons are lost more easily in barium / the attraction between the nucleus and outer electrons is weaker in barium
- The gas produced is hydrogen
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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