At GCSE you learned that reactivity increases down a group of metals as the outer electrons are lost more easily. A-Level Group 2 makes that quantitative — the trends in radius, ionisation energy and melting point, with reasons — and adds the characteristic Group 2 chemistry: reactions with water (where conditions change the products), and the solubility patterns of the hydroxides and sulfates that explain their uses in medicine and agriculture.
Trends down the group
Going down Group 2 from magnesium to barium, each element has one more electron shell than the last. That single fact drives the physical trends.
| Down Mg → Ba | Trend | Because… |
|---|---|---|
| Atomic radius | increases | each element has an extra electron shell, so the atom is larger |
| First ionisation energy | decreases | the outer electrons are further from the nucleus and more shielded, so easier to remove |
| Melting point | no simple trend — Mg is anomalously low, then it falls Ca→Ba | all giant metallic lattices, but Mg’s differs, so it is out of line; from Ca the metallic bonding weakens as the atoms enlarge |
| Reactivity | increases | losing the two outer electrons gets easier as ionisation energy falls |
Interactive — down the group, metal by metal
Pick a Group 2 metal to see how each extra shell changes its radius, ionisation energy and reactivity.
- Name the particles. The first ionisation energy falls because the outer electron is further from the nucleus and more shielded — not just “bigger atom”.
- Melting point needs the structure. Group 2 metals are giant metallic lattices; the trend is not perfectly smooth, so explain in terms of metallic bonding weakening as atoms enlarge.
- Successive ionisation energies jump when a shell is broken into — e.g. beryllium’s third ionisation energy is huge because it removes an electron from the inner 1s shell.
Recall the toolkit: atomic radius and first ionisation energy in Periodicity, successive ionisation energies in Atomic Structure, and metallic bonding in Bonding.
🧪 Exam-style questions
Explain why the first ionisation energy of the Group 2 elements decreases down the group.
Show answer
Down the group the atoms have more shells, so the outer electron is further from the nucleus and there is more shielding. 1 mark
The attraction between the nucleus and the outer electron is weaker, so less energy is needed to remove it. 1 mark
State why the atomic radius of calcium is greater than the atomic radius of magnesium.
Show answer
Calcium has more (electron) shells. 1 mark
Describe the structure and bonding in magnesium.
Show answer
A giant (metallic) lattice of Mg2+ ions 1 mark and delocalised electrons. 1 mark
“Metallic bonding” = the attraction between the positive ions and the delocalised electrons.
Source: AQA A-Level Chemistry past papers.
Reactions with water
The key here is one word: conditions. Magnesium behaves differently with cold water and with steam, and you need to know which product forms.
With cold water (very slow): magnesium hydroxide (sparingly soluble) and hydrogen.
Mg + 2H2O → Mg(OH)2 + H2
With steam (vigorous, a bright white light): magnesium oxide and hydrogen.
Mg + H2O → MgO + H2
Calcium, strontium and barium react with water more readily, and increasingly vigorously down the group (their ionisation energies are falling, so the metal is more reactive). Each gives the hydroxide and hydrogen:
Ca + 2H2O → Ca(OH)2 + H2
Barium is the most reactive of the four: dropped into water it fizzes steadily, giving a colourless solution (barium hydroxide is soluble) with effervescence of hydrogen.
Magnesium’s eagerness to lose electrons makes it a useful reducing agent. Titanium is extracted from titanium(IV) chloride by heating with magnesium:
TiCl4 + 2Mg → Ti + 2MgCl2
Magnesium (0 → +2) is oxidised and reduces titanium (+4 → 0) — a redox reaction.
Interactive — reactions with water
Pick a metal — and, for magnesium, the condition — to see the reaction, the products and the observation.
- Conditions change the product. Examiner reports flag wrong products — magnesium with cold water gives the hydroxide; with steam it gives the oxide.
- Use the accepted observation wording. Magnesium in steam burns with a bright white light; barium in water gives a colourless solution with effervescence — not just “it fizzes”.
- All four give hydrogen and the metal hydroxide with water; reactivity increases down the group as ionisation energy falls.
🧪 Exam-style questions
Which statement is correct? Tick (✓) one box.
State one observation when magnesium reacts with steam.
Show answer
A bright white light (and a white solid, magnesium oxide, is formed). 1 mark
Barium metal is added to a large excess of water. Which observation is correct and complete? Tick (✓) one box.
Source: AQA A-Level Chemistry past papers.
Solubility of hydroxides & sulfates
Two solubility trends run down Group 2 — and they run in opposite directions. Learn them together and they are hard to confuse.
| Down Mg → Ba | Trend | The end members |
|---|---|---|
| Hydroxides, M(OH)2 | solubility increases | Mg(OH)2 sparingly soluble → Ba(OH)2 soluble |
| Sulfates, MSO4 | solubility decreases | MgSO4 soluble → BaSO4 insoluble |
Because the hydroxides get more soluble down the group, their solutions release more OH− and become more alkaline from magnesium to barium. Because the sulfates get less soluble, barium sulfate is essentially insoluble — the fact behind both the sulfate test and the barium meal.
Interactive — solubility down the group
- The two trends run opposite ways. Hydroxide solubility increases down the group; sulfate solubility decreases. Mixing them up is the classic error.
- Use the right words: Mg(OH)2 is “sparingly soluble”; BaSO4 is “insoluble”.
Uses, the sulfate test & Required Practical 4
Those solubility trends explain most of Group 2’s everyday chemistry.
- Mg(OH)2 — an antacid (“milk of magnesia”): sparingly soluble, so mildly alkaline and safe to swallow, it neutralises excess stomach acid.
- Ca(OH)2 — spread on fields in agriculture to neutralise acidic soils and raise the pH.
- CaO or CaCO3 — used to remove SO2 from flue gases (flue-gas desulfurisation): CaO + SO2 → CaSO3. Where that SO2 comes from is the combustion story in Alkanes — combustion & pollutants.
- BaSO4 — the “barium meal” in medicine: it is opaque to X-rays, and because it is insoluble the toxic Ba2+ ions are not absorbed, so it is safe to swallow.
Barium sulfate’s insolubility also gives chemistry its standard test for sulfate ions:
RP4 is a set of test-tube reactions for cations (Group 2 ions and NH4+) and anions (halides, OH−, CO32− and SO42−). The sulfate test is below and the halide test is with Group 7; the rest:
- Group 2 cations — use the opposite solubility trends above. Add sodium hydroxide (hydroxides get more soluble down the group, so Mg2+ gives a white precipitate but Ba2+ stays in solution) or dilute sulfuric acid (sulfates get less soluble down the group, so Ba2+ gives a white precipitate but Mg2+ stays in solution).
- Ammonium, NH4+ — add sodium hydroxide and warm; the ammonia given off turns damp red litmus blue. NH4+ + OH− → NH3 + H2O.
- Hydroxide, OH− — turns red litmus blue (the solution is alkaline).
- Carbonate, CO32− — add dilute acid; it effervesces and the gas turns limewater milky. CO32− + 2H+ → CO2 + H2O.
Run the anion tests in order — carbonate, then sulfate, then halide — and acidify with nitric acid so you don’t add the ion you are testing for. Full method, apparatus, safety and exam questions are gathered on the required practicals page.
Add acidified barium chloride solution. A white precipitate of barium sulfate confirms a sulfate:
Ba2+(aq) + SO42−(aq) → BaSO4(s)
The barium chloride is acidified first with dilute HCl (or nitric acid) to remove carbonate and sulfite ions, which would otherwise also give a white precipitate and a false positive.
- State a precipitate or a solution — “a colour change” will not score. Give the observation in full: a white precipitate here, or a colourless solution in the water reactions. A precipitate is an insoluble solid that forms when two solutions are mixed.
- Acidify the barium chloride first. Examiner reports flag missing the acid step — it removes carbonate and sulfite ions that would also give a white precipitate.
- Acidify with dilute HCl or nitric acid — not sulfuric acid, which would add sulfate ions and ruin the test.
- BaSO4 is safe as a barium meal precisely because it is insoluble, so the toxic barium ions are not absorbed.
🧪 Exam-style questions
What is a use for barium sulfate? Tick (✓) one box.
Which statement is correct? Tick (✓) one box.
Source: AQA A-Level Chemistry past papers.
Capstone: down the group
Everything about Group 2 lines up with one idea — each step down adds a shell, so the atoms get bigger and lose their two outer electrons more easily.
| Down Mg → Ba | Direction | Consequence |
|---|---|---|
| Atomic radius | increases | outer electrons further out |
| First ionisation energy | decreases | metal more reactive |
| Reactivity with water | increases | from very slow (Mg) to vigorous (Ba) |
| Hydroxide solubility | increases | solutions more alkaline down the group |
| Sulfate solubility | decreases | BaSO4 insoluble — the sulfate test & barium meal |
Only two things break the smooth-trend picture, and both need extra care: the conditions for magnesium with water (cold water → hydroxide; steam → oxide), and the two solubility trends running opposite ways.
Interactive — true or false?
🧪 Exam-style questions
Explain why the third ionisation energy of beryllium is much higher than its second ionisation energy.
Show answer
The third electron is removed from the 1s sub-shell (an inner shell), rather than the 2s. 1 mark
This electron is closer to the nucleus and less shielded, so it is held much more strongly and needs far more energy to remove. 1 mark
Beryllium is 1s2 2s2: the first two electrons come from 2s, the third from the inner 1s.
Source: AQA A-Level Chemistry past papers.
- Down Mg→Ba: atomic radius increases (more shells); first ionisation energy decreases (outer electron further/more shielded); reactivity increases; melting point has no simple trend (Mg anomalously low, then falls Ca→Ba; all giant metallic).
- Mg + cold water → Mg(OH)2 + H2 (very slow). Mg + steam → MgO + H2 (vigorous, bright white light). The products depend on the conditions.
- Ca/Sr/Ba + water → M(OH)2 + H2, increasingly vigorous down the group (e.g. Ca + 2H2O → Ca(OH)2 + H2).
- Mg extracts titanium: TiCl4 + 2Mg → Ti + 2MgCl2 (Mg is the reducing agent).
- Hydroxide solubility increases down the group (Mg(OH)2 sparingly soluble → Ba(OH)2 soluble); sulfate solubility decreases (MgSO4 soluble → BaSO4 insoluble). The two trends run opposite ways.
- Uses: Mg(OH)2 antacid; Ca(OH)2 neutralises acidic soils; CaO/CaCO3 remove SO2 from flue gases; BaSO4 is the barium meal in X-rays (insoluble, so non-toxic).
- Sulfate test: add acidified BaCl2 → white precipitate of BaSO4. Acidify (HCl or HNO3, not H2SO4) to remove carbonate/sulfite ions that would also give a white precipitate.