Physical Property Trends

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

s-block elements occupy Groups 1 (alkali metals: Li, Na, K, Rb, Cs) and 2 (alkaline earth metals: Be, Mg, Ca, Sr, Ba) of the periodic table. Each has a low ionization energy because the outermost electron is shielded by filled inner shells and sits far from the nucleus, so they readily form M⁺ (Group 1) or M²⁺ (Group 2) cations with noble gas configuration. Down each group, atomic radius increases, first ionization energy decreases, and electronegativity falls, while melting point decreases sharply (weaker metallic bonding in heavier Group 1 metals). Solubility trends are opposite: Group 2 hydroxides become more soluble down the group, but Group 2 sulfates become less soluble — driven by lattice enthalpy decreasing faster than hydration enthalpy for hydroxides, but the reverse for sulfates. UNEB UACE Paper 1 (multiple choice) and Paper 2 (structured) typically test flame colours, thermal stability of carbonates/nitrates, and water hardness removal. Memorise: Li = crimson, Na = yellow, K = lilac, Ca = brick-red, Ba = green.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Physical Property Trends

Going down Group 1, melting point drops from Li (180 °C) to Cs (28 °C) because the metallic bond weakens as the cation–electron “sea” becomes more diffuse. Group 2 cations are smaller and doubly charged, so they polarise electron clouds more strongly — hence Be and Mg show partial covalent character, while Ca, Sr, Ba are genuinely ionic. First ionization energy decreases down each group (Li 520 kJ mol⁻¹ → Cs 376 kJ mol⁻¹; Be 899 → Ba 503) due to greater shielding and a larger atomic radius.

Reactions with Water, Oxygen and Chlorine

2 Na + 2 H₂O → 2 NaOH + H₂ (vigorous)
Mg reacts only with steam: Mg + H₂O(g) → MgO + H₂. Beryllium is inert to water owing to an oxide skin and high ionization energy.

Oxides, Hydroxides and Carbonates

Basicity of oxides and hydroxides increases down each group because the M–O bond becomes more ionic and the oxide ion is released more easily in water. Thermal stability of carbonates increases down Group 2: BeCO₃ decomposes at ~100 °C, while BaCO₃ requires >1300 °C. Cation charge density (polarising power) is highest for Be²⁺, distorting the CO₃²⁻ anion and weakening the C–O bond. For nitrates, Group 2 decompose as: 2 M(NO₃)₂ → 2 MO + 4 NO₂ + O₂, while Group 1 nitrates (except LiNO₃) release O₂ and form nitrites: 2 NaNO₃ → 2 NaNO₂ + O₂.

Solubility Contrasts

Compound	Trend down Group 2	Reason
Hydroxides, M(OH)₂	Solubility increases	Lattice enthalpy drops faster than hydration enthalpy
Sulfates, MSO₄	Solubility decreases	Hydration enthalpy drops faster; BaSO₄ is virtually insoluble
Carbonates, MCO₃	Solubility decreases	Lattice enthalpy dominates

Diagonal Relationships

Li resembles Mg (both form nitrides Li₃N, Mg₃N₂; both give carbonates that decompose readily; both form covalent organometallics). Be resembles Al (both amphoteric — BeO and Al₂O₃ dissolve in NaOH(aq); both form covalent halides that dimerise).

Worked Stoichiometry

EDTA titration of water hardness: Ca²⁺(aq) + Y⁴⁻(aq) → [CaY]²⁻(aq). Because 1 mol EDTA ≡ 1 mol Ca²⁺, hardness in mg L⁻¹ CaCO₃ = (V_EDTA × M_EDTA × 100.09 × 1000) / V_sample. A 25.0 cm³ sample requiring 18.40 cm³ of 0.0100 M EDTA gives (18.40 × 0.0100 × 100.09 × 1000)/25.0 = 736.7 mg L⁻¹ CaCO₃.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Hydration vs Lattice Enthalpy — the Quantitative Argument

Solubility depends on ΔG_solution = ΔH_lattice + ΔH_hydration + TΔS. For Group 2 hydroxides, the lattice is held together by 2+ / 1− attractions, but going down the group the cation grows so the lattice energy falls steeply; the smaller, highly-charged Be²⁺ grips OH⁻ tightly, hence Be(OH)₂ is only sparingly soluble. For sulfates, the SO₄²⁻ anion is large and polarisable, so the lattice with a small cation like Mg²⁺ is actually stabilised by polarisation, while hydration of the same cation is strongly exothermic. Net result: MgSO₄ is very soluble, but the lattice of BaSO₄ is so dominant that BaSO₄ is precipitated quantitatively in qualitative analysis.

Water Hardness and Removal

Ugandan borehole water frequently contains Ca(HCO₃)₂ — temporary hardness removed simply by boiling: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g). Permanent hardness from CaCl₂ or CaSO₄ is removed by adding Na₂CO₃ (washing soda): CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2 NaCl, or by ion-exchange resins that swap Ca²⁺/Mg²⁺ for Na⁺/H⁺. Boiler scale (CaSO₄, CaCO₃) is dangerous because it reduces heat transfer and may crack heated tubes — a common UACE structured question theme.

Flame Tests — Handling the Sodium Mask

Sodium’s intense yellow line at 589 nm masks the lilac K⁺ emission. UACE practicals therefore specify viewing through cobalt blue glass, which absorbs the Na yellow and transmits the K violet. Always clean the nichrome wire in concentrated HCl before each test to avoid cross-contamination.

Common Examination Pitfalls

Saying all Group 1 carbonates decompose on heating — only Li₂CO₃ behaves like a Group 2 carbonate; Na₂CO₃ and K₂CO₃ are thermally stable to >800 °C.
Treating Be as a typical Group 2 metal: it is amphoteric, forms covalent BeCl₂ (linear, polymeric in solid), and does not react with cold water.
Confusing nitrate decomposition stoichiometry — the molar ratio of NO₂:O₂ is 4:1 for Group 2, but 0:1 (only O₂) for Group 1 (excluding Li).

Adjacent Links

Topic 3 connects forward to electrode potentials (E° for Li⁺/Li = –3.04 V makes Li a powerful reducing agent despite its high ionization energy, because of its very negative hydration enthalpy) and qualitative analysis (Group II cations in the cation systematic scheme are Mg, Ca, Sr, Ba).

Practice Prompts

(P1) State and explain how the thermal stability of the carbonates changes from MgCO₃ to BaCO₃. (3 marks)
(P2) 20.0 cm³ of well water required 14.50 cm³ of 0.00500 M EDTA for total hardness. Calculate the hardness in mg L⁻¹ CaCO₃ and classify the water as soft (<60), moderately hard (60–120), hard (120–180) or very hard (>180 mg L⁻¹).

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Physical Property Trends