In simple terms
A friendly intro before the formal notes — no formulas yet.
The Alkaline Earth Trendsetters
Group 2 metals show predictable trends down the group because their atoms get larger and lose electrons more easily. This affects everything from how they react with water to the properties of the compounds they form.
Imagine a tall tower of magnets with a steel ball on top. The taller the tower (the further down the group), the weaker the magnetic pull (nuclear attraction) on the ball (valence electron), making it much easier to knock off (ionise).
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Reactivity increases down Group 2. This is because ionisation becomes easier, a factor that outweighs changes in lattice and hydration enthalpies.
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The general equations are M + 2H₂O → M(OH)₂ + H₂ and M + 2HCl → MCl₂ + H₂. Observations, like fizzing rate, are more vigorous for barium than magnesium.
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Thermal stability of carbonates/nitrates increases down the group. The larger cation at the bottom of the group has a lower charge density and polarises the anion less.
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Solubility trends are crucial: hydroxides become more soluble down the group, while sulfates become less soluble. This makes BaSO₄ an insoluble salt.
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Key formulas
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Full topic notes
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Atomic Properties and Reactivity
As we descend Group 2, from Mg to Ba, each element gains an additional electron shell. This has two major consequences: the atomic radius increases, and the shielding of the outer electrons by inner shells increases. Although the nuclear charge also increases, the combined effects of increased distance and shielding mean the electrostatic attraction between the nucleus and the two valence electrons weakens. Consequently, the first and second ionisation energies decrease down the group. Since chemical reactions for these metals involve losing these two electrons to form a 2+ ion, a lower ionisation energy means the element is more reactive.
Atomic Radius: Increases down the group (more electron shells).
Shielding: Increases down the group (more inner electrons).
Ionisation Energy: Decreases down the group (weaker nuclear attraction for valence electrons).
Reactivity: Increases down the group (easier to form M²⁺ ions).
Solubility of Group 2 Compounds
The solubility of Group 2 compounds shows two opposing trends that are a common focus in exams. To dissolve an ionic lattice, energy is needed to break the lattice (lattice enthalpy) and energy is released when the resulting gaseous ions are hydrated (hydration enthalpy). The overall enthalpy change of solution determines solubility. For Group 2, we see two distinct patterns.
Hydroxides, M(OH)₂: Solubility increases down the group. Magnesium hydroxide, , is sparingly soluble (used in indigestion remedies as 'milk of magnesia'), whereas barium hydroxide, , is much more soluble, forming a strongly alkaline solution.
Sulfates, MSO₄: Solubility decreases down the group. Magnesium sulfate, ('Epsom salts'), is very soluble, but solubility drops sharply, and barium sulfate, , is famously insoluble.
You must know these two opposing solubility trends. A simple way to remember is 'SOH-DAH': Solubility Of Hydroxides Down A Hill (increases), and sulfates are the opposite. The insolubility of barium sulfate is the basis for the test for sulfate ions.
Thermal Stability of Carbonates and Nitrates
Thermal stability refers to how resistant a compound is to breaking down upon heating. For Group 2 carbonates and nitrates, stability increases down the group. This trend is explained by the concept of polarising power. The M²⁺ cation attracts the electron cloud of the large, polyatomic anions (carbonate, , or nitrate, ). A small cation with a high charge density (like ) is highly polarising and distorts the anion's electron cloud significantly. This distortion weakens the covalent bonds within the anion, making it easier to break apart upon heating. As you go down the group, the M²⁺ cation gets larger, its charge density decreases, and its polarising power weakens. Therefore, the anion is less distorted, and the compound is more thermally stable.
Decomposition of carbonates:
Decomposition of nitrates:
Trend: Thermal stability of both carbonates and nitrates increases down Group 2.
Reason: Cation size increases down the group, leading to lower charge density and reduced polarising power.
Effect: The larger cations (e.g., ) distort the anion ( or ) less, so more energy (a higher temperature) is required for decomposition.
Worked examples
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Explain, in terms of atomic structure, why barium is more reactive than magnesium.
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Barium is further down Group 2 than magnesium, so it has more electron shells.
A student adds aqueous sodium sulfate, , to a test tube containing magnesium chloride solution, . They repeat the experiment in a second test tube containing barium chloride solution, . Describe the expected observations and write an ionic equation for any reaction that occurs.
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Test tube 1 (with ): Observation: No change / no precipitate is formed. Explanation: Magnesium sulfate () is soluble in water.
How it all connects
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Glossary
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Quick check
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Revision flashcards
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What is the general trend in first ionisation energy down Group 2?
It decreases. The increasing atomic radius and shielding effect outweigh the increase in nuclear charge, making it easier to remove the outermost electron.
Key takeaways
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Atomic Radius: Increases down the group (more electron shells).
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Shielding: Increases down the group (more inner electrons).
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Ionisation Energy: Decreases down the group (weaker nuclear attraction for valence electrons).
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Reactivity: Increases down the group (easier to form M²⁺ ions).
Practice — then mark it
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Test Your Knowledge on Group 2 Properties
Test Your Knowledge on Group 2 Properties
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