Ionic Bonding

• Ionic bonds form by electron transfer from metal to non-metal forming cations and anions.
• Lattice energy and factors affecting ionic compound stability (charge, size).
• Born–Haber cycle concept for formation enthalpy of ionic solids (NCERT qualitative).
• Properties of ionic compounds — high mp, brittleness, conductivity in melt/aqueous solution.

Level 1 — Formation and structure

Verbal: A metal with low ionisation energy loses electrons; a non-metal with high electron affinity gains them. Oppositely charged ions attract in a 3D crystal lattice (not isolated Na⁺Cl⁻ pairs in solid).

Symbolic: Lattice energy U ∝ z⁺z⁻/(r⁺ + r⁻); formula from charge balance (e.g., Ca²⁺ + 2 Cl⁻ → CaCl₂).

Example: Na + ½Cl₂ → NaCl(s); Na → Na⁺ + e⁻; Cl + e⁻ → Cl⁻.

Formula from valence: Cross charges — Na⁺ Cl⁻ → NaCl; Ca²⁺ and Cl⁻ → CaCl₂.

Lewis: Show transfer; draw ions with octets (or 18-electron for some cations).

Level 2 — Lattice energy and properties

Lattice energy (U): Energy released when gaseous ions form solid lattice — larger |charge| and smaller ions → higher U → higher melting point.

Polarising power / polarisability (Fajan's rules preview): Small highly charged cation distorts anion electron cloud → partial covalent character in some "ionic" compounds (e.g., AgCl less soluble).

Common ionic examples: NaCl, MgO, CaF₂, NH₄⁺ salts (polyatomic cation).

NCERT spotlight — Born-Haber cycle qualitatively

Formation of NaCl(s) from elements involves sublimation of Na, ionisation of Na, dissociation of Cl2, electron affinity of Cl, and lattice formation. Most exothermic step is often lattice energy release.

Solubility trend: Lattice energy and hydration energy compete. Smaller ions often give higher lattice energy; hydration also depends on charge density.

Ionic character from electronegativity: Delta EN large favours ionic bonding; Fajans rules predict covalent character when cation is small and highly charged.

Predict formula and write formation for magnesium and oxygen. Compare lattice strength with NaCl qualitatively.

Step 1 — Mg (2 valence e⁻) → Mg²⁺; O (6 valence) → O²⁻ (needs 2 e⁻).
Step 2 — Formula: MgO (charge balance 2+ and 2−).
Step 3 — 2Mg + O₂ → 2MgO — oxide coating protects Mg metal.
Step 4 — Mg²⁺ smaller, higher charge than Na⁺; O²− vs Cl⁻ higher charge → MgO lattice energy >> NaCl.
Step 5 — MgO mp ≈ 2852 °C vs NaCl ≈ 801 °C — consistent with stronger ionic lattice.

Applications — lattice energy and melting point

MgO higher melting point than NaCl reflects doubled charges on ions (Mg2+, O2-) and smaller ionic radii — stronger electrostatic lattice. Born-Lande equation qualitatively: U proportional to z+ z- / r — guides predicting hardness and solubility trends in ionic crystals for comparative inorganic questions.

Deep dive — lattice energy and Born-Haber cycle

Born-Haber cycle for NaCl(s): ΔH_f = ΔH_sub(Na) + IE1(Na) + ½D(Cl2) − EA(Cl) − U_lattice — algebraic sum of steps from elements to solid. Most negative contribution typically lattice energy U releasing energy when gaseous ions form crystal. Kapustinskii equation approximate U proportional z+ z− / (r+ + r−) — smaller ions higher charge stronger lattice. Hydration enthalpy competes with lattice energy in dissolution — if hydration exceeds lattice, salt soluble (Group 1 nitrates). Fajans rules: polarising cation (small, high charge) distorts anion electron cloud → partial covalency, lower melting point, colour (AgCl vs NaCl). Ionic radii trends: isoelectronic series O2− F− Na+ Mg2+ Al3+ — same electrons, increasing nuclear charge shrinks radius. Defects (non-stoichiometric compounds) advanced — NCERT mentions ideal lattice; real crystals have vacancies affecting conductivity coloured centres.

Review and practice drill

Review checklist: (1) Electron transfer metal to nonmetal. (2) Lattice energy increases with charge, decreases with size. (3) Conduct molten/aqueous not solid. (4) Formula from charge balance. Practice: Al3+ and O2- give Al2O3.

• Write Lewis formation of Ca²⁺ and F⁻ leading to CaF₂.
• Why do ionic solids not conduct when solid?
• Which has higher lattice energy: NaF or NaI? Why?

Open the Practice tab for graded questions on Ionic.