Detailed and comprehensive notes on group 2 (unit 3.2.2 of AQA AS level chemistry). Covers group 2 trends, reactions with water, compound solubility and uses of group 2 elements.
Group 2 - Trends
Atomic Radius Increases down the Group
• The greater atomic number means more electrons are placed into shells increasingly further away from the
nucleus.
• (Atoms of group 2 are smaller than equivalent group 1 atoms due to extra nuclear charge.)
First Ionisation Energy Decreases down the Group
• Increased shielding offsets increased nuclear charge, so the major variant is the distance between the
outermost electron and the nucleus.
• This distance increases, so the attraction between the outermost electron and the nucleus decreases.
• (Successive ionisation energy values get larger and decrease down the group.)
Melting Points Decrease down the Group
• Group 2 elements have metallic structures and each atom contributes 2 electrons to the delocalised sea.
• Down the group, the distance between the delocalised electrons and the nucleus increases, so attraction
between them decreases.
• (Melting points of group 2 elements are higher than similar group 1 elements because of a higher number of
delocalised electrons.)
Reaction of Magnesium with Oxygen
• Mg burns in oxygen with a bright white flame.
• 2Mg + O2 → 2MgO
• MgO is a white solid with a high melting point due to its strong ionic bonding.
Reaction of Group 2 Elements with Water
When group 2 elements react, they are oxidised from a state of 0 to +2, forming M2+ ions:
Magnesium
• Magnesium reacts in steam to produce magnesium oxide and hydrogen.
• Mg burns with a bright white flame.
• Mg + H2O → MgO + H2
• Magnesium also react with warm water to form magnesium hydroxide.
• Mg + 2H2O → Mg(OH)2 + H2
• This reaction is much slower and there is no flame.
Calcium
• Ca + 2H2O → Ca(OH)2 + H2
• Reacts with cold water.
• Fizzes in water and white precipitate forms (Ca(OH)2 = slightly soluble).
Strontium
• Sr + 2H2O → Sr(OH)2 + H2
• Fizzes in water and forms colourless solution (Sr(OH)2 = soluble).
Barium
• Ba + 2H2O → Ba(OH)2 + H2
• Fizzes in water and forms colourless solution (Ba(OH)2 = soluble).
Going down the group, the fizzing would become more vigorous, the metal would dissolve faster and the solution
would heat up more.
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