Ionic Bond

An ionic bond forms when a metal loses electrons and a nonmetal gains them, so cations and anions stick by electrostatic attraction. Coulomb’s law says attraction grows as distance shrinks, and lattice energy rises with larger charges and smaller ionic radii. Ionic crystals arrange positive and negative ions in a regular lattice: insulators as solids, conductors when molten or in solution. Here you change ion distance to feel the Coulomb force and inspect the NaCl crystal lattice.

How Does an Ionic Bond Form?

Ion distance3

💡 Analogy: Magnet N and S Poles

①Na (metal) loses one electron → Na⁺ cation

②Cl (non-metal) gains one electron → Cl⁻ anion

③Cation and anion bind by electrostatic attraction

④Closer distance → stronger attraction (1/r²)

Coulomb's Law and Lattice Energy

Coulomb's Law

F = kq₁ q₂r²

q: ionic charge, r: ion distance, k: Coulomb constant

Lattice Energy

U ∝ |q₁||q₂| / r

Measure of ionic bond strength — larger means stronger bond

🔍 Lattice Energy and Physical Properties

①Larger ionic charge → larger lattice energy (MgO > NaCl)

②Smaller ionic radius → larger lattice energy

③Larger lattice energy → higher melting/boiling point

④Ionic compounds = high mp + crystalline structure

Structure of Ionic Crystals

💡 Properties of Ionic Compounds

①Crystal: cations and anions in a regular arrangement

②High mp/bp (NaCl: 801℃)

③Insulator in solid state

④Conductor when molten or in solution (free ion motion)

⑤Brittle — like-charge layers repel when shifted

Work It Out

Example 1

Find the chemical formula of the ionic compound made of sodium ions (Na⁺) and oxide ions (O²⁻).

Identify the charge of each ion.

Na⁺ charge +1, O²⁻ charge −2

Adjust the ion ratio so the total charge is 0.

total charge 0 → Na : O = 2 : 1 → Na₂O

▸ Na₂O

Two +1 ions and one −2 ion give a net charge of 0, yielding a stable compound.

Example 2

When magnesium (Mg) and chlorine (Cl) form an ionic bond, find the resulting formula and each ion.

Find the ion each element forms.

Mg → Mg²⁺ + 2e⁻, Cl + e⁻ → Cl⁻

Balance the charges to determine the formula.

charge balance (+2)+2(−1)=0 → MgCl₂

▸ MgCl₂ (Mg²⁺, Cl⁻)

One +2 ion is offset by two −1 ions, so two Cl are needed.

Summary

Ionic Bond Formation

metal + non-metal → cation + anion → ionic bond

Forms between elements with large electronegativity difference

2022 KICE mock Chemistry I type, adapted

What is the formula of the ionic compound made of aluminum ions (Al³⁺) and oxide ions (O²⁻)?

①AlO

②Al₂O₃

③Al₃O₂

④AlO₂

⑤Al₂O

▸ ② Al₂O₃

Find the smallest integer ratio a : b satisfying charge balance (+3)×a + (−2)×b = 0.

Using the least common multiple 6 gives a=2, b=3.

(+3)×2 + (−2)×3 = 0 → Al₂O₃

🎯 Exam Points

①Ionic bond = metal + non-metal (large EN difference)

②Na⁺ is smaller than Na, Cl⁻ is larger than Cl

③Lattice energy ∝ product of charges / ion distance

④Ionic: insulator as solid, conductor in solution/melt

⑤Write the cation first in a formula (NaCl, MgO)

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Isotopes & Atomic Mass

Covalent Bond

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