Magnesium barely reacts with cold water; barium fizzes vigorously. Every Group 2 metal loses the same two electrons, but larger atoms give them up more easily, so reactivity climbs down the group.
All Group 2 elements have two outer s electrons and form M2+ ions, so their compounds have oxidation number +2. Down the group the atoms gain shells, so atomic radius increases and shielding increases; the outer electrons are less strongly held, so the first and second ionisation energies decrease. Less energy is needed to form M2+, so reactivity increases from Mg to Ba. The metals burn in oxygen to form MO, react with water to give hydroxides and hydrogen (Mg only slowly unless it is steam), and react with dilute acids to form salts and hydrogen.
Move from magnesium to barium in the simulator and compare the size of each atom and how tightly its outer electrons are held.
| Mg | Ca | Sr | Ba | |
|---|---|---|---|---|
| outer electrons | 3s2 | 4s2 | 5s2 | 6s2 |
| atomic radius / nm | 0.160 | 0.197 | 0.215 | 0.217 |
| 1st IE / kJ mol−1 | 738 | 590 | 550 | 503 |
The extra shells add distance and shielding, which outweigh the larger nuclear charge.
Run each reaction in the simulator and compare the vigour from magnesium to barium.
| Reaction | Equation | Observation |
|---|---|---|
| burn in oxygen | 2Mg + O2 → 2MgO | Mg burns with a brilliant white flame; white solid |
| Mg with steam | Mg + H2O(g) → MgO + H2 | very slow with cold water; burns in steam |
| Ca, Sr, Ba with water | Ca + 2H2O → Ca(OH)2 + H2 | fizzing, increasingly vigorous down the group; alkaline solution |
| with dilute HCl | Mg + 2HCl → MgCl2 + H2 | effervescence, faster down the group |
| oxide with water | CaO + H2O → Ca(OH)2 | exothermic; alkaline solution |
Calcium, strontium and barium react with dilute sulfuric acid at first but soon slow down, because their sulfates are insoluble and coat the metal. Do not use this to argue that reactivity decreases down the group. And magnesium with steam gives the oxide, not the hydroxide.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
How does the first ionisation energy change down Group 2?
Which equation shows magnesium reacting with steam?
Which Group 2 metal reacts most vigorously with water?
What is the oxidation number of calcium in all its compounds?
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Why does reactivity increase down Group 2?
Calcium is added to water. Which statement is correct?
Barium reacts with dilute sulfuric acid quickly at first but the reaction soon stops. Why?
What volume of hydrogen, at room conditions, is produced when 0.411 g of barium reacts completely with water? (Ar Ba 137.3; molar volume 24.0 dm3 mol−1)
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