# Group 2 properties and reactions

> CIE A-Level Chemistry · 9701
> Source: https://www.owlsprep.com/study/cie-9701-u10-group-2-properties-and-reactions/

This subtopic covers key physical and chemical trends of Group 2 (alkaline earth) metals, their reactions with common reagents, and atomic-level explanations for periodic trends, all core content for CIE A-Level Chemistry.

**Prerequisites:** [Atomic structure and electron configuration](https://www.owlsprep.com/study/cie-9701-u03-atomic-structure-and-periodicity/); Ionization energy periodic trends

## Learning objectives

- Describe key physical and chemical trends down Group 2
- Explain trends in terms of atomic structure and ionization energy
- Balance equations for common Group 2 reactions
- Predict solubility and thermal stability trends for Group 2 compounds

## Physical Trends down Group 2

**Alkaline Earth Metals** — All Group 2 elements are s-block metals with 2 valence electrons in their outermost s orbital, forming +2 ions in almost all stable compounds.

*Notation:* Group 2

*Example:* Calcium forms $Ca^{2+}$, barium forms $Ba^{2+}$

As you descend Group 2, the number of electron shells increases, leading to predictable changes in key physical properties:

| Property | Trend down Group 2 | Explanation |
| --- | --- | --- |
| Atomic radius | Increases | More electron shells, increased shielding outweighs higher nuclear charge |
| First ionization energy | Decreases | Valence electrons are further from the nucleus, weaker electrostatic attraction |
| Melting point | Generally decreases | Larger atomic radius reduces the strength of metallic bonding |
| Density | Generally increases | Increase in atomic mass outweighs increased atomic volume |

**Worked example:** Predict and explain the difference in first ionization energy between magnesium and strontium.

1. Strontium is below magnesium in Group 2, so it has more electron shells than magnesium.
2. This gives strontium a larger atomic radius and more inner electron shielding of the valence electrons.
3. The attraction between the nucleus and valence electrons is weaker in strontium, so less energy is required to remove an electron.
4. Therefore, the first ionization energy of strontium is lower than that of magnesium.

> **Exam tip:** Always link trend explanations to atomic structure (shielding, nuclear charge, atomic radius) for full marks.

## Reactions with Oxygen and Water

Reactivity of Group 2 increases down the group, as lower ionization energy means valence electrons are lost more easily in redox reactions.

All Group 2 metals react with oxygen to form solid oxides, with the general equation:

$$2M(s) + O_2(g) \rightarrow 2MO(s)$$

**Worked example:** Write the balanced equation for the reaction of calcium metal with excess oxygen gas.

1. Calcium is a Group 2 metal, forms $Ca^{2+}$ ions, so its neutral oxide is $CaO$.
2. Write the unbalanced equation:
3. $$Ca(s) + O_2(g) \rightarrow CaO(s)$$
4. Balance oxygen atoms by adding a coefficient of 2 to $CaO$, then balance calcium by adding a coefficient of 2 to $Ca$:
5. $$2Ca(s) + O_2(g) \rightarrow 2CaO(s)$$
6. Check that all atoms are balanced, which they are, so this is the final equation.

For reaction with water, rate increases down the group: magnesium reacts very slowly with cold water, but rapidly with steam, while barium reacts vigorously with cold water. The general equation for reaction with cold water is:

$$M(s) + 2H_2O(l) \rightarrow M(OH)_2(s/aq) + H_2(g)$$

> **warning**
>
> Magnesium reacts with steam to form solid magnesium oxide ($MgO$), not magnesium hydroxide. This is a common exam trap.

## Reactions with Acids and Solubility Trends

All Group 2 metals react with dilute acids to form a salt and hydrogen gas, with reaction rate increasing down the group. The general equation for reaction with hydrochloric acid is:

$$M(s) + 2HCl(aq) \rightarrow MCl_2(aq) + H_2(g)$$

**Worked example:** Write the balanced equation for the reaction of magnesium with dilute sulfuric acid, and name the products.

1. Magnesium forms $Mg^{2+}$, the sulfate ion is $SO_4^{2-}$, so the neutral salt is magnesium sulfate ($MgSO_4$).
2. Unbalanced equation:
3. $$Mg(s) + H_2SO_4(aq) \rightarrow MgSO_4(aq) + H_2(g)$$
4. Check that all atoms are balanced: 1 Mg, 2 H, 1 S, 4 O on both sides.
5. Products are aqueous magnesium sulfate and hydrogen gas.

Two key solubility trends for Group 2 compounds are commonly tested:

- Solubility of hydroxides **increases** down Group 2
- Solubility of sulfates **decreases** down Group 2

> **Solubility memory hook**
>
> "Hydroxides High up Low, Sulfates Slow down Low" → Hydroxides: Low solubility top, High solubility bottom; Sulfates: High solubility top, Low solubility bottom.

## Thermal Stability of Group 2 Compounds

**Thermal stability** — The resistance of a compound to decomposition when heated.

Thermal stability of Group 2 carbonates and nitrates increases down the group. A smaller Group 2 cation has higher charge density, which polarizes the anion (carbonate or nitrate) more strongly, weakening bonds within the anion and making decomposition easier.

The general equation for thermal decomposition of Group 2 carbonates is:

$$MCO_3(s) \rightarrow MO(s) + CO_2(g)$$

**Worked example:** Explain why magnesium carbonate decomposes at a lower temperature than barium carbonate.

1. $Mg^{2+}$ is a smaller cation than $Ba^{2+}$, so $Mg^{2+}$ has a higher charge density.
2. The higher charge density of $Mg^{2+}$ polarizes the $CO_3^{2-}$ anion more strongly, weakening the bonds within the carbonate ion.
3. Less energy is required to break the bonds and decompose $MgCO_3$, so it decomposes at a lower temperature than $BaCO_3$.

## Common pitfalls

- **Wrong:** Stating that melting point always increases down Group 2
  - Why it fails: While density increases, melting point generally decreases as larger atomic size weakens metallic bonding
  - Correct: State that melting point generally decreases down Group 2
- **Wrong:** Writing $Mg(OH)_2$ as a product of magnesium reacting with steam
  - Why it fails: Hot steam dehydrates any formed hydroxide to the oxide
  - Correct: Write $MgO(s) + H_2(g)$ as products for Mg + steam
- **Wrong:** Claiming all Group 2 hydroxides are insoluble in water
  - Why it fails: Solubility increases down the group, heavier hydroxides are highly soluble
  - Correct: Remember that hydroxide solubility increases down Group 2
- **Wrong:** Stating thermal stability of carbonates decreases down Group 2
  - Why it fails: Smaller cations have higher polarizing power that destabilizes the carbonate ion
  - Correct: State that carbonate thermal stability increases down Group 2
- **Wrong:** Writing Group 2 ions as +1 when balancing equations
  - Why it fails: All common Group 2 compounds have +2 cations, this leads to incorrect balancing
  - Correct: Always use +2 charge for Group 2 ions when balancing reaction equations

## Cheatsheet

| Trend | Direction down Group 2 |
| --- | --- |
| Atomic radius | Increases |
| First ionization energy | Decreases |
| Melting point | Decreases |
| Reactivity | Increases |
| Hydroxide solubility | Increases |
| Sulfate solubility | Decreases |
| Carbonate thermal stability | Increases |

## What's next

The periodic trends and reaction patterns you learned for Group 2 form a critical foundation for studying other groups in the periodic table, including Group 17 halogens which frequently react with Group 2 compounds. Trends in solubility of Group 2 compounds are core to qualitative analysis tests for common anions like sulfate, and thermal decomposition trends are often tested in practical exam questions. Understanding how atomic structure dictates bulk properties is also a key transferable skill for all inorganic chemistry topics.

- [Group 17 properties and reactions](https://www.owlsprep.com/study/cie-9701-u10-group-17-properties-and-reactions/)
- [Nitrogen and sulfur](https://www.owlsprep.com/study/cie-9701-u11-overview/)
- [Nitrogen and its compounds](https://www.owlsprep.com/study/cie-9701-u11-nitrogen-and-its-compounds/)

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