Group 2: The Alkaline Earth Metals

This guide covers subtopic 10.1, Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds, from Topic 10 of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is AS Level content. A2 Topic 27 revisits Group 2 with quantitative lattice-energy and enthalpy-of-hydration explanations — none of that appears here; this resource stays entirely qualitative, as the AS syllabus requires.

Before studying this

This resource assumes the trend-explaining habits from The Periodic Table: Periodicity Across Period 3 — particularly reasoning from ionic charge and radius — and nuclear charge/shielding from Atomic Structure: Particles, Radius and Isotopes. Group 2 (magnesium to barium) is a down-a-group trend, distinct from the across-a-period trends covered there.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 10

10.1 Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds — describing and writing equations for the reactions of the elements with oxygen, water and dilute hydrochloric and sulfuric acids; describing and writing equations for the reactions of the oxides, hydroxides and carbonates with water and dilute hydrochloric and sulfuric acids; describing and writing equations for the thermal decomposition of the nitrates and carbonates, including the trend in thermal stability; describing and predicting the trends in physical and chemical properties of the elements and compounds involved in the above; stating the variation in solubility of the hydroxides and sulfates.

Reactions of the elements

Group 2 metals (M = Mg, Ca, Sr, Ba) become more reactive down the group — larger atoms lose their two outer electrons more easily, since those electrons are further from the nucleus and more shielded.

• With oxygen: 2M + O₂ → 2MO (magnesium burns with a characteristic brilliant white light).
• With water: M + 2H₂O → M(OH)₂ + H₂. Magnesium reacts only very slowly with cold water (readily with steam); calcium reacts steadily with cold water; strontium and barium react vigorously — reactivity with cold water clearly increases down the group.
• With dilute hydrochloric acid: M + 2HCl → MCl₂ + H₂.
• With dilute sulfuric acid: M + H₂SO₄ → MSO₄ + H₂ — except that barium’s reaction stops almost immediately, because BaSO₄ is insoluble and coats the metal surface, physically blocking further acid contact. This is a useful exam point: it’s a solubility effect, not a reactivity one.

Reactions of the oxides, hydroxides and carbonates

With water: MO + H₂O → M(OH)₂. Solubility increases down the group (see below), so this reaction becomes more complete — MgO gives only a sparingly-soluble, weakly alkaline suspension, while BaO reacts to give a much more strongly alkaline solution.

With dilute acid (oxide, hydroxide and carbonate all follow the same logic — a base or a carbonate reacting with acid):

MO + 2HCl → MCl₂ + H₂O

M(OH)₂ + 2HCl → MCl₂ + 2H₂O

MCO₃ + 2HCl → MCl₂ + H₂O + CO₂

(and equivalently with dilute H₂SO₄, again with barium’s reaction limited by the insolubility of BaSO₄).

Thermal decomposition of nitrates and carbonates

Every Group 2 carbonate and nitrate decomposes on heating:

MCO₃ → MO + CO₂

2M(NO₃)₂ → 2MO + 4NO₂ + O₂

(the brown gas NO₂ and the glowing splint/relit-splint response of O₂ are useful observational confirmation of this reaction).

Thermal stability increases down the group — barium’s carbonate and nitrate need a higher temperature to decompose than magnesium’s. The qualitative explanation (as required at AS, without invoking lattice energy): a smaller, more highly charged cation polarises the large carbonate or nitrate anion more strongly, distorting its electron cloud and weakening the bonds within it, which makes it easier to break apart on heating. Mg²⁺ is small and polarises strongly, so MgCO₃ decomposes at a relatively low temperature; Ba²⁺ is large and polarises weakly, so BaCO₃ needs much more heating before it decomposes.

Worked example. Predict, with a reason, which decomposes at a lower temperature: CaCO₃ or SrCO₃.

Ca²⁺ is smaller than Sr²⁺ (fewer electron shells), so it has a higher charge density and polarises the carbonate ion more strongly, weakening its bonds more. CaCO₃ decomposes at the lower temperature.

Solubility trends

This is the single most-tested fact in this topic, and it runs in opposite directions for hydroxides and sulfates — worth learning as a pair, precisely because it’s easy to mix them up:

• Hydroxides become more soluble down the group: Mg(OH)₂ is only sparingly soluble (a suspension, weakly alkaline); Ba(OH)₂ is considerably more soluble, giving a strongly alkaline solution.
• Sulfates become less soluble down the group: MgSO₄ is freely soluble; BaSO₄ is essentially insoluble — the basis of the qualitative sulfate-ion test (adding aqueous BaCl₂ and dilute HCl to a solution gives a dense white precipitate of BaSO₄ if sulfate ions are present) and of using barium sulfate as an X-ray contrast medium, since it doesn’t dissolve in the body.

Common mistakes

• Applying the same solubility direction to both hydroxides and sulfates. They trend in opposite directions down the group — check which one a question is asking about before answering.
• Explaining thermal stability using lattice energy or hydration enthalpy. That quantitative treatment is A2 content (Topic 27). At AS, the required explanation is qualitative: cation size and charge density affecting how strongly the anion is polarised.
• Assuming barium reacts fully with dilute sulfuric acid, like it does with hydrochloric acid. The insoluble BaSO₄ coating stops the reaction early — a solubility effect worth stating explicitly if asked to compare the two acids.
• Forgetting that “more reactive down the group” and “more soluble down the group” are two separate trends describing different things (the metal’s reactivity vs. a compound’s solubility) that happen to both increase down the group for some compounds but not others.

Quick revision checklist

• Reactivity of the elements with oxygen, water and dilute acids: increases down the group
• Reactions of oxides, hydroxides and carbonates with water and dilute acids, with equations
• Thermal decomposition of nitrates and carbonates: equations, and increasing stability down the group (polarisation explanation, no lattice energy)
• Solubility: hydroxides more soluble down the group; sulfates less soluble down the group
• Barium sulfate’s insolubility: the sulfate test, and why Ba reacts only briefly with dilute H₂SO₄

Related resources

• The Periodic Table: Periodicity Across Period 3 — the same polarisation/charge-density reasoning applied across a period
• Group 17: The Halogens — a second group studied in the same way
• Group 2: Quantitative Trends in Thermal Stability and Solubility — the A Level energetics explanation for these trends
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.