Chapter 12: The Group 2 Elements

Part A: The Essentials

In this first section we summarize the key features of the chemistry of the Group 2 elements.

Alkaline Earth Metals

The elements calcium, strontium, barium, and radium are known as the alkaline earth metals, but the term is often applied to the whole of Group 2. All the elements are silvery white metals and the bonding in their compounds is normally described in terms of the ionic model.

Some aspects of the chemical properties of beryllium are more like those of a metalloid with a degree of covalence in its bonding. The elements are denser, harder, and less reactive than the elements of Group 1 but are still more reactive than many typical metals.

12.1 The Elements

Key Point: The most important factors influencing the chemical properties of the Group 2 elements are their ionization energies and ionic radii.

Be

Beryllium

r = 112 pm | I₁ = 900 kJ/mol

Mg

Magnesium

r = 150 pm | I₁ = 736 kJ/mol

Ca

Calcium

r = 197 pm | I₁ = 590 kJ/mol

Sr

Strontium

r = 215 pm | I₁ = 548 kJ/mol

Ba

Barium

r = 217 pm | I₁ = 502 kJ/mol

Ra

Radium

r = 220 pm | I₁ = 510 kJ/mol

Selected Properties

Property	Be	Mg	Ca	Sr	Ba	Ra
Metallic radius/pm	112	150	197	215	217	220
Ionic radius r(M²⁺)/pm	27(4)	72(6)	100(6)	126(8)	142(8)	170(12)
First ionization energy/kJ mol⁻¹	900	736	590	548	502	510
E°(M²⁺, M)/V	−1.85	−2.38	−2.87	−2.89	−2.90	−2.92
Density ρ/g cm⁻³	1.85	1.74	1.54	2.62	3.51	5.00
Melting point/°C	1280	650	850	768	714	700
Δ_hydH°(M²⁺)/kJ mol⁻¹	−2500	−1920	−1650	−1480	−1360	–

Ionization Energy Trends

First Ionization Energy (kJ/mol)

Be

900

Mg

736

Ca

590

Sr

548

Ba

502

Ra

510

Natural Occurrence

Beryllium: Semiprecious mineral beryl, Be₃Al₂(SiO₃)₆
Magnesium: 8th most abundant in Earth's crust; extracted from seawater and dolomite
Calcium: 5th most abundant; occurs as limestone, marble, chalk, and biominerals
Sr, Ba: Extracted by electrolysis of molten chlorides
Radium: All isotopes radioactive; extracted from uranium-bearing minerals

Reactivity with Water

M(s) + 2 H₂O(l) → M(OH)₂(aq) + H₂(g)

Ca, Sr, Ba, Ra react readily with cold water; Mg reacts only with hot water.

Flame Test Colors

Ca

Orange-red

Sr

Crimson

Ba

Yellowish-green

Ra

Deep red

12.2 Simple Compounds

Key Point: The binary compounds of the Group 2 metals contain the cations of the elements and exhibit predominantly ionic bonding.

All elements occur as M(II) with ns² valence configuration. Apart from Be, compounds are predominantly ionic.

Hydrides

Group 2 elements (except Be) form ionic hydrides containing H⁻. BeH₂ has a 3D network of linked BeH₄ tetrahedra. MgH₂ is studied as a hydrogen storage material.

Halides

Formed by direct combination. Fluorides of larger cations adopt the fluorite structure (8:4), MgF₂ has rutile structure (6:3). Be halides form covalent networks.

Oxides & Hydroxides

BeO has wurtzite structure (4:4); others adopt rock-salt (6:6). BaO dissolves in water forming strongly basic solution:

BaO(s) + H₂O(l) → Ba²⁺(aq) + 2OH⁻(aq)

Be(OH)₂ is amphoteric; forms [Be(OH)₄]²⁻ in strong base.

Carbides

Be₂C contains C⁴⁻ (methide); MC₂ (others) contains C₂²⁻ (acetylide):

CaC₂(s) + 2H₂O(l) → Ca(OH)₂(s) + C₂H₂(g)

12.3 The Anomalous Properties of Beryllium

Key Point: Compounds of beryllium show a high level of covalency and beryllium shows a strong diagonal relationship with aluminium.

Small Be²⁺ (27 pm) has high charge density → largely covalent compounds, strong Lewis acid. Coordination number typically 4, tetrahedral geometry.

Consequences of Small Size

Covalent bonding in halides and hydride
Complex formation (e.g., Be₄O(O₂CCH₃)₆)
Hydrolysis forming [Be(OH₂)₃OH]⁺ and acidic solutions
4:4-coordination structures for oxide/chalcogenides
Stable organometallic compounds

Be ↔ Al Diagonal Relationship

Similarities with Aluminium

Both form covalent hydrides and halides
Both oxides are amphoteric
Both form [M(OH)₄]ⁿ⁻ complexes
Both form structures based on linked tetrahedra
Both carbides contain C⁴⁻ and produce methane with water
Both alkyl compounds are electron-deficient with M–C–M bridges

12.4 Occurrence and Extraction

Key Point: Magnesium is the only Group 2 element extracted on an industrial scale; Mg, Ca, Sr, Ba can be extracted from molten chloride.

Magnesium from Seawater

A litre of seawater contains >1 g Mg²⁺. Extraction relies on Mg(OH)₂ being less soluble than Ca(OH)₂:

Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s)

Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)

Then electrolysis of molten MgCl₂.

Calcium

5th most abundant element. The name comes from Latin calx meaning 'lime'. Average adult human contains ~1 kg calcium. Central to biominerals and cell signalling.

Radium

Discovered by Pierre and Marie Curie in 1898. Pitchblende contains ~1 g Ra in 10 t of ore. The Curies took 3 years to isolate 0.1 g RaCl₂.

12.5 Uses of the Elements and Their Compounds

Key Point: Magnesium and its compounds have major applications in pyrotechnics, alloys, and common medicines; calcium compounds are widely used in construction.

Beryllium Uses

Light alloys for precision instruments, aircraft, missiles
X-ray tube windows (transparent to X-rays)
Nuclear moderator

⚠️ Toxicity Warning

BeO is extremely toxic and carcinogenic by inhalation, causing chronic beryllosis.

Magnesium Uses

Light alloys with aluminium for construction/aircraft
Fireworks and flares (intense white flame)
Milk of Magnesia Mg(OH)₂—indigestion remedy
Epsom Salts MgSO₄·7H₂O—constipation, sprains
MgO—refractory furnace lining
Grignard reagents in organic synthesis

Calcium Uses

CaO (lime)—mortar, cement, steelmaking
CaSO₄·2H₂O (gypsum)—plasterboard
CaSO₄—drying agent
CaCO₃—Solvay process
CaF₂—IR/UV spectrometer windows

Strontium, Barium, Radium

Sr—pyrotechnics (red), phosphors
Ba—X-ray imaging (BaSO₄ barium meals)
BaCO₃—glassmaking, rat poison
BaSO₄—drilling muds, spectroscopy standard
Ra—cancer treatment (historically)

📦 Box 12.1: Cement and Concrete

Limestone + aluminosilicates heated to 1500°C → clinker → Portland cement.

Hydration produces Ca₃Si₂O₇·H₂O and Ca(OH)₂. Alkali silicate reaction causes cracking in ageing structures.

🎆 Box 12.2: Fireworks and Flares

Green: BaCl⁺ | Red: SrCl⁺ | White: Mg

Distress flares use Sr(NO₃)₂ with sawdust, waxes, sulfur, KClO₄.

12.6 Hydrides

Key Point: All Group 2 elements form saline hydrides except beryllium, which forms a polymeric covalent compound.

Ionic hydrides react with water:

MgH₂(s) + 2H₂O(l) → Mg(OH)₂(s) + 2H₂(g)

Hydrogen Storage

MgH₂ contains 7.7 wt% H. The process Mg + H₂ ⇌ MgH₂ is reversible above 250°C. Research focuses on reducing decomposition temperature through doping with transition metals and nanoparticle formation.

📦 Box 12.3: Hydrogen Storage Materials

Δ_rH = +74.4 kJ/mol due to high MgH₂ lattice enthalpy (2718 kJ/mol).

Subnanometer clusters show reduced decomposition temperature (~200°C for small clusters).

12.7 Halides

Key Point: Be halides are covalent; all fluorides except BeF₂ are insoluble; other halides are soluble.

BeCl₂ Structures

Solid: Polymeric chain (sp³, tetrahedral)
Vapour: Dimer (sp²)
>900°C: Linear monomer (sp)

Fluoride Structures

Compound	Structure	Coordination
BeF₂	Quartz-like	4:2
MgF₂	Rutile	6:3
CaF₂, SrF₂, BaF₂	Fluorite	8:4

Road De-icing

CaCl₂ more effective than NaCl: exothermic dissolution (−82 kJ/mol), minimum freezing point −55°C, less toxic to plants.

12.8 Oxides, Sulfides, and Hydroxides

Oxides

BeO: wurtzite, m.p. 2570°C, highest thermal conductivity, toxic.

Others: rock-salt structure. CaO thermoluminescent ("limelight").

Peroxides decompose: MO₂ → MO + ½O₂ (stability increases down group).

Complex Oxides

Perovskite: BaTiO₃ (ferroelectric)
Spinel: MgAl₂O₄
Superconductors: YBa₂Cu₃O₇

Hydroxides

Solubility increases down the group: Mg(OH)₂ sparingly soluble → Ba(OH)₂ soluble (strongly basic).

Limewater test:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

12.9 Nitrides and Carbides

Key Point: Nitrides and carbides react with water to produce ammonia and either methane or ethyne.

M₃N₂(s) + 6H₂O(l) → 3M(OH)₂ + 2NH₃(g)

Be₂C(s) + 4H₂O(l) → 2Be(OH)₂(s) + CH₄(g)

CaC₂(s) + 2H₂O(l) → Ca(OH)₂(s) + C₂H₂(g)

💡 Historical Note

Calcium carbide was widely used in vehicle lights (enabling safe night driving) and miners' lamps in the late 19th century.

12.10 Salts of Oxoacids

Carbonates

Decomposition temperature: Mg (350°C) → Ba (1360°C)

CaCO₃ occurs as limestone, chalk, marble; used in construction and as agricultural lime.

Solubility in CO₂-containing water forms stalactites/stalagmites:

CaCO₃(s) + H₂O(l) + CO₂(g) ⇌ Ca²⁺(aq) + 2HCO₃⁻(aq)

Sulfates

Gypsum CaSO₄·2H₂O used in fireproof wallboard.

Plaster of Paris CaSO₄·½H₂O expands when mixed with water.

BaSO₄ for X-ray imaging, drilling muds, spectroscopy standard.

Water Hardness

Temporary: Ca/Mg hydrogencarbonates (removed by boiling)

Permanent: Ca/Mg sulfates (removed by ion exchange)

12.11 Solubility, Hydration, and Beryllates

Key Point: Large negative hydration enthalpies of salts with mononegative ions ensure solubility. For dinegative ions, lattice enthalpies dominate → insolubility.

BeF₂ exception: very high hydration enthalpy of small Be²⁺:

BeF₂(s) + 4H₂O(l) → [Be(OH₂)₄]²⁺(aq) + 2F⁻(aq) Δ_rH° = −250 kJ/mol

Beryllates

Amphoteric Be forms [Be(OH)₄]²⁻. Beryllates built from BeO₄ tetrahedra:

Beryl family: Be₃Al₂(SiO₃)₆ (emerald, aquamarine)
Chrysoberyl: BeAl₂O₄ (alexandrite changes color)

12.12 Coordination Compounds

Key Point: Only beryllium forms coordination compounds with simple ligands; most stable complexes form with polydentate chelating ligands like edta.

Basic Beryllium Acetate

Be₄O(O₂CCH₃)₆: Central O surrounded by tetrahedron of Be atoms, bridged by ethanoate ions.

EDTA Complexes

Formation constants: Ca²⁺ > Mg²⁺ > Sr²⁺ > Ba²⁺

Biological Complexes

Chlorophylls: Mg porphyrin complexes (photosynthesis)
Mg: Phosphate transfer, carbohydrate metabolism
Ca: Biominerals, cell signalling, muscle action

12.13 Organometallic Compounds

Key Point: Alkylberyllium compounds polymerize in solid phase; Grignard reagents are among the most important main-group organometallic compounds.

Beryllium Organometallics

Pyrophoric in air, unstable in water. Be(CH₃)₂ is monomer in vapour (linear), polymer in solid (3c,2e bridging).

Grignard Reagents

Mg(s) + RBr(sol) → RMgBr(sol)

Used as R⁻ sources in organic synthesis. Schlenk equilibria: R₂Mg, RMgX, MgX₂ all present.

Mg(I) Compound

LMg−MgL with Mg−Mg bond (285 pm)—shorter than in Mg metal (320 pm)!

Exercises & Further Reading

📝 Chapter Exercises

Explain why Be compounds are mainly covalent while others are ionic.
Why is Be more similar to Al and Zn than to Mg?
Why does BeF₂ form a glass when cooled from melt?
Calculate H wt% in Group 2 hydrides. Why is MgH₂ studied for H₂ storage?
Which is more soluble: MgSeO₄ or BaSeO₄?
How can Ra be separated from other Group 2 cations?