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Chemistry 3% exam weight

Periodic Properties and Group Chemistry

Part of the JAMB UTME study roadmap. Chemistry topic chem-17 of Chemistry.

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Periodic Properties and Group Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Periodic properties are the recurring trends in atomic and ionic radii, ionisation energy (IE), electron affinity (EA), and electronegativity (χ) that arise from changing effective nuclear charge (Z_eff) across periods and groups. Z_eff = Z − σ, where σ is the shielding constant from Slater’s rules.

Across Period 3, oxide/hydroxide acidity shifts: Na₂O/MgO basic → Al₂O₃ amphoteric → SiO₂ weakly acidic → P₄O₁₀, SO₃, Cl₂O₇ acidic. Down Group 1, reactivity, basicity of MOH, and ionic radius all increase; flame colours are Li = crimson, Na = yellow, K = lilac, Rb = red, Cs = blue.

Down Group 17, oxidising power and electronegativity fall (F₂ > Cl₂ > Br₂ > I₂), but hydrohalic acid strength rises: HF is weak, HCl/HBr/HI are strong. Only AgF is soluble among silver halides; AgCl (white), AgBr (pale yellow), AgI (yellow) precipitate.

  • IE1 jumps appear at Group 2→13 (s→p transition) and Group 15→16 (paired p-electron repulsion).
  • Electronegativity (Pauling): F ≈ 3.98 is the maximum; χ falls down a group.
  • Inert pair effect stabilises +1 in heavy Group 13 (Tl) and +2 in heavy Group 14 (Sn, Pb).

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Atomic and Ionic Radii

Atomic radius decreases across a period (Z_eff rises, shielding nearly constant) and increases down a group (extra shell outweighs Z_eff gain). Cations are smaller than their parent atoms; anions are larger. For isoelectronic species (e.g. Na⁺, Mg²⁺, Al³⁺), radius shrinks as nuclear charge rises.

Ionisation Energy, Electron Affinity, Electronegativity

First IE generally rises across a period with two notable discontinuities: a dip at Group 13 (new p-subshell, weaker hold) and a dip at Group 15→16 (paired-electron repulsion in the 2p orbital of O, S). Successive IEs rise sharply — a large jump marks removal of a core electron. EA becomes more exothermic across a period up to Cl; noble gases have positive EA.

Mulliken scale: χ_M = (IE + EA)/2, with IE and EA in the same units (eV). Convert to Pauling by dividing by 5.6 eV (≈ 5.6 eV ≈ 2.54 × 10⁻¹⁹ J).

Period 3 Oxide–Hydroxide Trend

OxideNa₂OMgOAl₂O₃SiO₂P₄O₁₀SO₃Cl₂O₇
NatureBasicBasicAmphotericWeakly acidicAcidicAcidicAcidic
HydroxideNaOH (strong base)Mg(OH)₂ (weak base)Al(OH)₃ amphotericH₂SiO₃ weak acidH₃PO₄ mediumH₂SO₄ strongHClO₄ very strong

Group 1 and Group 2 Reactivity

Alkali metals (M) react with water to give MOH + ½H₂; reactivity intensifies down the group as IE1 falls. Alkaline earths show the same trend but with +2 oxidation and a steady fall in thermal stability of carbonates down the group — a classic JAMB trap, because basicity of hydroxides increases down the group.

Basicity vs thermal stability: Group 2 hydroxide basicity rises down the group, but carbonate/nitrate thermal stability falls down the group.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Halogens and the Silver Halide Trap

Down Group 17, bond energy falls (HF exceptionally strong from H-bonding), so oxidising power falls: F₂ > Cl₂ > Br₂ > I₂. Acid strength of HX rises down the group because the H–X bond weakens and the larger X⁻ is stabilised by charge dispersal. The solubility of silver halides is a frequent UTME question:

  • AgF: soluble (no precipitate with AgNO₃).
  • AgCl: white precipitate, soluble in dilute NH₃.
  • AgBr: pale yellow, soluble in concentrated NH₃.
  • AgI: yellow, insoluble in NH₃.

Inert Pair Effect and Diagonal Relationships

In heavier p-block metals, the ns² electrons resist ionisation, leaving the (n−1)d¹⁰ ns² “inert pair”. This stabilises Tl⁺ over Tl³⁺, Pb²⁺ over Pb⁴⁺, and Bi³⁺ over Bi⁵⁺. Diagonal relationships (Li/Mg, Be/Al, B/Si) come from similar charge/size ratios: LiOH and Mg(OH)₂ are both weakly basic; BeCl₂ and AlCl₃ are both covalent and polymeric.

Born–Haber Connection

Lattice energy scales as U ∝ (Z⁺Z⁻)/(r⁺ + r⁻). This explains why MgO (small r⁺, r⁻, +2 charge) has a far higher lattice enthalpy than NaCl, and why MgO’s higher charge density raises its melting point dramatically compared with Na₂O.

Practice Prompts

  1. Arrange Li⁺, Na⁺, K⁺, Rb⁺ in order of increasing ionic radius and justify using Z_eff and shell count.
  2. Explain why Al(OH)₃ dissolves in both NaOH(aq) and HCl(aq), writing equations for each reaction.

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