Ionic bonding and structure

IB Chemistry HL· 2.1 Ionic bonding and structure· 35 min read

1. Formation of Ionic Bonds★★☆☆☆⏱ 10 min

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📘 Definition

Ionic bond

Electrostatic attraction between oppositely charged ions formed by electron transfer between elements with large electronegativity differences (usually >1.7 on the Pauling scale)

Example:

Bond between $Na^{+}$ and $Cl^{-}$ in sodium chloride

Ionic bonding typically occurs between a metal (low ionization energy, loses electrons easily) and a non-metal (high electron affinity, gains electrons easily). The metal forms a positive cation, the non-metal forms a negative anion, and electrostatic attraction holds the ions together.

📐 Worked Example

Show the formation of magnesium oxide from magnesium and oxygen atoms via electron transfer.

1
Write the ground-state electron configuration of each neutral atom:
2
$Mg: 1s^2 2s^2 2p^6 3s^2 \\ O: 1s^2 2s^2 2p^4$
3
Identify electrons lost/gained to form stable full outer shells:
4
$Mg \rightarrow Mg^{2+} + 2e^- \\ O + 2e^- \rightarrow O^{2-}$
5
Oppositely charged ions are attracted electrostatically to form neutral magnesium oxide, with a full ionic bond.

Exam tip:

Always explicitly state the attraction is electrostatic between oppositely charged ions for full marks; 'opposite charges attract' is not sufficient.

2. Formula of Ionic Compounds★★★☆☆⏱ 15 min

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All neutral ionic compounds have zero net charge, so total positive charge from cations equals total negative charge from anions. We can use this rule to deduce the simplest whole-number (empirical) formula.

📘 Definition

Empirical formula (ionic)

The simplest whole-number ratio of cations to anions in the giant ionic lattice

📐 Worked Example

Deduce the formula of aluminum sulfate, where aluminum forms $Al^{3 +}$ and sulfate is the polyatomic ion $SO_{4}^{2 -}$ .

1
Find the lowest common multiple of the absolute charge values: $∣ + 3∣ \times ∣ - 2∣ = 6$
2
Calculate the number of each ion needed for zero net charge: 2 $Al^{3 +}$ give $2 \times (+ 3) = + 6$ , 3 $SO_{4}^{2 -}$ give $3 \times (- 2) = - 6$ . Total charge = 0.
3
Write the formula, using parentheses around polyatomic ions if there is more than one: $Al_{2} (SO_{4})_{3}$

✓ Quick check

Test your understanding of ion formulas

What is the formula of calcium phosphate, where calcium is $Ca^{2 +}$ and phosphate is $PO_{4}^{3 -}$ ?
- $CaPO_{4}$
- $Ca_{3} (PO_{4})_{2}$
- $Ca_{2} PO_{4}$
- $Ca (PO_{4})_{2}$
Reveal answer
1 —
3 calcium ions give $3 \times (+ 2) = + 6$ , 2 phosphate ions give $2 \times (- 3) = - 6$ , for a net charge of zero.

3. Giant Ionic Lattice Structure★★★☆☆⏱ 15 min

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Ionic compounds do not form discrete small molecules. Instead, ions arrange into a regular, repeating 3D structure called a giant ionic lattice, where every ion is surrounded by oppositely charged ions. This arrangement maximizes attractive electrostatic forces and minimizes repulsion.

📐 Worked Example

Explain why a cation has a smaller ionic radius than its parent neutral atom, while an anion has a larger radius than its parent.

1
For a cation: The metal loses all valence electrons to form the ion, so an entire electron shell is lost. The same nuclear charge acts on fewer electrons, pulling the remaining electron cloud closer to the nucleus, decreasing radius.
2
For an anion: The non-metal gains extra electrons to form the ion, so nuclear charge stays the same while electron count increases. Increased electron-electron repulsion expands the electron cloud, increasing radius.

4. Physical Properties of Ionic Compounds★★☆☆☆⏱ 10 min

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High melting/boiling points: Strong electrostatic attraction between ions requires large amounts of energy to break the lattice.
Electrical conductivity: Only conduct when molten or dissolved in water, not when solid. Solid lattices have fixed ions (no mobile charge carriers).
Brittleness: Applying force shifts ion layers, bringing like charges next to each other; repulsion splits the crystal.
Solubility: Most ionic compounds dissolve in polar solvents like water, which hydrate individual ions and break the lattice.

📐 Worked Example

Explain why solid sodium chloride does not conduct electricity, but molten sodium chloride does.

1
In solid sodium chloride, $Na^{+}$ and $Cl^{-}$ ions are held in fixed positions in the giant ionic lattice by strong electrostatic attraction. There are no free mobile charge carriers to carry an electric current.
2
When sodium chloride melts, the ionic bonds are broken and ions become free to move throughout the liquid. Mobile charged ions can carry an electric current, so molten NaCl conducts.

Exam tip:

Always link the observed physical property directly to ionic lattice structure to earn full marks in IB exams.

5. Common Pitfalls

Wrong move:

Stating ionic bonding is just "attraction between ions" without mentioning electrostatic attraction

Why:

IB exams require explicit reference to the electrostatic nature of the bond for full marks

Correct move:

Always describe ionic bonding as the electrostatic attraction between oppositely charged ions

Wrong move:

Writing calcium hydroxide as $CaOH_{2}$ instead of $Ca(OH)_{2}$

Why:

Omitting parentheses around polyatomic ions leads to ambiguity about the number of ions

Correct move:

Use parentheses around polyatomic ions when more than one is present in the formula

Wrong move:

Claiming ionic compounds conduct electricity via free electrons

Why:

This confuses ionic bonding with metallic bonding; ionic compounds have no delocalized free electrons

Correct move:

State conductivity comes from mobile ions when molten or dissolved in water

Wrong move:

Assuming all ionic compounds are soluble in water

Why:

Many common ionic compounds (e.g. silver chloride, barium sulfate) are insoluble

Correct move:

State most ionic compounds are soluble in polar solvents, but solubility varies based on lattice and hydration energy

Wrong move:

Thinking ionic compounds only contain two elements

Why:

Polyatomic ions (e.g. nitrate, ammonium) form ionic compounds with 3+ elements

Correct move:

Recognize ionic compounds can contain multiple elements when polyatomic ions are present

6. Quick Reference Cheatsheet

Concept	Key Exam Fact
Ionic bond	Electrostatic attraction between oppositely charged ions
General formation	Metal (loses e⁻ → cation) + Non-metal (gains e⁻ → anion)
Common ion charges	Group 1: +1, Group 2: +2, Group 15: -3, Group 16: -2, Group 17: -1
Structure	Giant ionic lattice, no discrete molecules
Melting point	High: lots of energy to break strong ionic attraction
Solid conductivity	No: no mobile charge carriers
Molten/aqueous conductivity	Yes: mobile charged ions carry current

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · Paper 1
Ionic lattice conductivity question
2024 · Paper 2
Deduce ionic compound formula
2023 · Paper 1
Ionic bond definition

What's Next

Ionic bonding is the first core bonding model you will learn in IB Chemistry HL, and it forms the foundation for all subsequent topics in this unit and the rest of the syllabus. You will build on this knowledge to compare ionic properties with covalent and metallic bonding, and extend it to HL topics like lattice enthalpy and Born-Haber cycles. Mastery of ionic compound formulas is also essential for stoichiometric calculations, writing net ionic equations, and balancing redox reactions later in the course.