What a learner can do afterwards
- Explains the reactivity trend down the group in terms of ionisation energy
- Writes equations for group 2 metals and their oxides reacting with water
- States the opposite solubility trends of the hydroxides and of the sulfates
- Explains a use such as barium sulfate in medical imaging or magnesium hydroxide in indigestion remedies from those trends
1 · Read
Down Group 2, atoms have more shells. The outer electrons are farther from the nucleus and more shielded. Ionisation energy falls, so the metals lose electrons more easily. Reaction with water becomes more vigorous from magnesium to barium. A metal makes a hydroxide and hydrogen: Ca + 2H2O -> Ca(OH)2 + H2. Magnesium reacts only slowly with cold water, but with steam it gives magnesium oxide and hydrogen: Mg + H2O -> MgO + H2.
The solubility patterns go in opposite directions. Group 2 hydroxides become more soluble down the group. Group 2 sulfates become less soluble, so barium sulfate dissolves very little.
A group 2 oxide reacts with water to make the hydroxide: CaO + H2O -> Ca(OH)2. Barium sulfate is used in gut scans because very little dissolves, so most passes through. Magnesium hydroxide dissolves enough to react with excess stomach acid.
Use distance, shielding and ionisation energy to explain reactivity. Do not use atom size alone. More distance and shielding mean a weaker pull on the outer electrons.
Reactivity and hydroxide solubility rise down Group 2, while sulfate solubility falls. These patterns explain the water reactions and the medical uses.
Original chemistry lesson and assessment text authored for os-taxonomy.
The reading on this page is original writing for os-taxonomy.
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