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*Revision Materials and Past Papers* 1 Atomic Structure 2 Amounts of Substance 3 Bonding 4 Energetics 5 Kinetics 6 Chemical Equilibria & Kc 7 Redox Equations 8 Thermodynamics 9 Rate Equations 10 Kp (Equilibrium Constant) 11 Electrode Potentials & Cells 12 Acids and Bases 13 Periodicity 14 Group 2: Alkaline Earth Metals 15 Group 7: The Halogens 16 Period 3 Elements & Oxides 17 Transition Metals 18 Reactions of Ions in Aqueous Solution 19 Intro to Organic Chemistry 20 Alkanes 21 Halogenoalkanes 22 Alkenes 23 Alcohols 24 Organic Analysis 25 Optical Isomerism 26 Aldehydes & Ketones 27 Carboxylic Acids & Derivatives 28 Aromatic Chemistry 29 Amines 30 Polymers 31 Amino Acids, Proteins & DNA 32 Organic Synthesis 33 NMR Spectroscopy 34 Chromatography RP1–RP12 Required Practicals

2.2 Group 2, The Alkaline Earth Metals

2.2 Group 2, The Alkaline Earth Metals

Group 2, The Alkaline Earth Metals

Specification Reference Inorganic chemistry, Group 2, the alkaline earth metals 3.2.2.1

Quick Notes

  • Atomic radius increases down Group 2 due to more electron shells.
  • First ionisation energy decreases down Group 2 due to increased shielding and weaker attraction between outermost electron and nucleus.
  • Melting points generally decrease down Group 2 (except for magnesium, which has an anomalous structure).
  • Group 2 metals react with water to form hydroxides and hydrogen; reactivity increases down the group.
  • Magnesium is used to extract titanium from TiCl4:
    TiCl4 + 2Mg → Ti + 2MgCl2.
  • Hydroxide solubility increases down the group (Mg(OH)2 sparingly soluble).
  • Sulfate solubility decreases down the group (BaSO4 insoluble; used in medical imaging).
    • Acidified BaCl2 is used to test for sulfate; acidifying prevents false positives from carbonates.

Full Notes

Group 2 Trends in Atomic Radius, First Ionisation Energy, and Melting Points

Atomic radius increases down the group:

Diagram showing atomic radius increasing down Group 2 due to additional shells

This is because additional electron shells increase shielding and the outermost electron shell is further from the nucleus.


First ionisation energy decreases down the group.

First ionisation energy = energy required to remove 1 mole of electrons from 1 mole of gaseous atoms.

Graph/illustration of decreasing first ionisation energy down Group 2

More electron shells increase shielding, reducing the attraction between the positive nucleus and the outer (negative) electrons. The outer electron is further from the nucleus, so it is easier to remove.


Melting points generally decrease down the group.

Trend plot showing Group 2 melting points generally decreasing down the group with Mg anomaly

Group 2 metals have a metallic structure: positive metal ions in a lattice are attracted to delocalised electrons (metallic bonding). Down the group the ions get larger, but the charge stays 2+, so the attraction between metal ions and delocalised electrons becomes weaker.

Bonding schematic linking weaker metallic bonding to lower melting points down Group 2

Less energy is required to overcome the attraction and melt the lattice, so melting points fall. Magnesium is an exception due to its particular metallic crystal structure.


Reactions of Group 2 Metals with Water

Group 2 metals react with water to form a metal hydroxide and hydrogen gas. Reactivity increases down the group because ionisation energy decreases.

General equation: M + 2H2O → M(OH)2 + H2

Group 2 metals reacting with water to form hydroxide and hydrogen

Magnesium in the Extraction of Titanium

Magnesium is used to extract titanium from titanium(IV) chloride (TiCl₄).

The magnesium reduces titanium(IV) chloride (TiCl4) at high temperature in an inert atmosphere to produce titanium metal (a redox process).

Mg is oxidised to Mg2+ while Ti is reduced from +4 to 0:

Overall reaction: TiCl4 + 2Mg → Ti + 2MgCl2

Schematic of magnesium reducing TiCl4 to titanium metal and MgCl2

The MgCl2 formed is water-soluble and can be separated from titanium.

Note - Titanium is found as TiO2 (rutile). It is first converted to TiCl4 by heating with chlorine and carbon; then TiCl4 is reduced by Mg.

Route from TiO2 to TiCl4 before reduction with Mg

Titanium cannot be extracted with carbon because TiC (titanium carbide) would form instead of the metal.


Solubility Trends of Group 2 Hydroxides

Hydroxide solubility increases down the group.

Bar/line graphic showing increasing solubility of Group 2 hydroxides down the group

Mg(OH)2 is sparingly soluble, whereas Ba(OH)2 is highly soluble.

The pH of group 2 metal hydroxide solutions increases down the group as the greater the solubility of the hydroxide, the greater the concentration of hydroxide (OH-) ions in the solution.

Ca(OH)2 (slaked lime) is used in agriculture to neutralise acidic soils.


Solubility Trends of Group 2 Sulfates

Sulfate solubility decreases down the group.

Trend showing decreasing solubility of Group 2 sulfates down the group

BaSO4 is essentially insoluble and is used in medical imaging (barium meals). Although barium ions are toxic, in BaSO4 the barium is locked in an insoluble compound and is not absorbed by the body.

Illustration of BaSO4 used in radiography because it is insoluble

Testing for Sulfate Ions Using BaCl2

BaCl2(aq) is used to test for sulfate ions (SO42−).

Ba2+(aq) reacts with SO42−(aq) to form BaSO4(s), a white precipitate.

Flow showing acidified BaCl2 test yielding white BaSO4 precipitate with sulfate ions

Ionic equation: Ba2+(aq) + SO42−(aq) → BaSO4(s)

Why is the BaCl2 acidified? Adding dilute HCl or HNO3 beforehand removes carbonate impurities (CO32−) that would otherwise also give a white precipitate (BaCO3) and cause a false positive.


Uses of Group 2 Compounds

Compound Use Reason
Mg(OH)2 Antacid in medicine Neutralises stomach acid
Ca(OH)2 Agriculture Neutralises acidic soil
CaO / CaCO3 Removes SO2 from flue gases Reacts with SO2 to form gypsum (CaSO4)
BaSO4 Medical X-rays Insoluble, non-toxic contrast agent

Flue Gas Desulfurisation:
CaO and CaCO3 are used to remove SO2 from industrial waste gases, reducing acid rain formation.

CaO + SO2 → CaSO3

CaCO3 + SO2 → CaSO3 + CO2

Summary

Idea or Concept Key Point Equation / Example
Atomic radius Increases down the group (more shells, more shielding). —
First ionisation energy Decreases down the group (weaker nuclear attraction to outer electron). —
Melting points Generally decrease down the group; Mg is anomalous (structure). Weaker metallic bonding as ion size increases.
Reaction with water Forms hydroxide and H2; reactivity increases down the group. M + 2H2O → M(OH)2 + H2
Mg extracts Ti Mg reduces TiCl4 to titanium metal. TiCl4 + 2Mg → Ti + 2MgCl2
Hydroxide solubility Increases down the group; pH of solutions increases. Mg(OH)2 (low) → Ba(OH)2 (high)
Sulfate solubility Decreases down the group; BaSO4 insoluble (barium meal). —
Sulfate test Acidified BaCl2 gives white BaSO4 precipitate; acid removes CO32−. Ba2+ + SO42− → BaSO4(s)