it follows that the formation of ionic compounds would primarily depend upon: • The ease of formation of the positive and negative ions from the respective neutral atoms; • The arrangement of the positive and negative ions in the solid, that is, the lattice of the crystalline compound.
-- NCERT Class 11 Chemistry, Ch. 4, p. 106Ionic Bonding Lattice
8 MCQs9-step worked example
Source: NCERT Chemical Bonding and Molecular StructureOfficial key: NTA-verifiedLast updated: 10 Oct 2026
Try this first
What gives a qualitative measure of the stability of an ionic compound?
- A.Achieving an octet of electrons around the ions in the gaseous state
- B.The enthalpy of lattice formation
- C.The ionization enthalpy of the metal alone
- D.The electron gain enthalpy of the non-metal alone
Tap to see the answer
Answer: B. B is correct. NCERT: “Thus a qualitative measure of the stability of an ionic compound is provided by its enthalpy of lattice formation and not simply by achieving octet of electrons around the ionic species in gaseous state.” (NCERT Class 11 Chemistry, Chapter 4, page 107).
A is wrong: A is wrong because NCERT says stability is measured by lattice formation enthalpy and not simply by achieving an octet around the gaseous ions (trap: octet equals stability).
C is wrong: C is wrong because ionization enthalpy is only one of the contributions to forming the ions. It does not measure the stability of the compound.
D is wrong: D is wrong because electron gain enthalpy is likewise only one contribution. The lattice enthalpy is the measure NCERT gives.
Ionic Bonding Lattice, explained for NEET
Sodium and chlorine atoms: the ionization enthalpy of Na (495.8 kJ mol⁻¹) and the electron gain enthalpy of Cl (−348.7 kJ mol⁻¹) add up to a positive 147.1 kJ mol⁻¹. So why does NaCl form at all? The answer is the lattice, and it is the point NEET questions probe.
The numbers. The sum 147.1 kJ mol⁻¹ is unfavourable, yet it is more than compensated by the enthalpy of lattice formation of NaCl(s), −788 kJ mol⁻¹. NCERT's wording: the crystal gets stabilised by the energy released when the lattice forms (NCERT Class 11 Chemistry, Chapter 4, page 106).
What decides whether an ionic compound forms. Two things (NCERT Class 11 Chemistry, Chapter 4, page 106):
- the ease of forming the positive and negative ions from the neutral atoms, and
- the arrangement of the ions in the solid, the lattice of the crystalline compound.
Ionic bonds form more easily between elements with comparatively low ionization enthalpies and elements with comparatively high negative electron gain enthalpy.
The solid. Ionic compounds in the crystalline state are orderly three-dimensional arrangements of cations and anions held together by coulombic interaction energies. The structure depends on ion size, packing and other factors; NaCl is the rock salt structure (page 106).
Lattice enthalpy. It is the energy required to completely separate one mole of a solid ionic compound into gaseous constituent ions. For NaCl it is 788 kJ mol⁻¹: 788 kJ separates one mole of NaCl(s) into one mole of Na⁺(g) and one mole of Cl⁻(g) to an infinite distance (NCERT Class 11 Chemistry, Chapter 4, page 107).
Stability and the octet. A qualitative measure of stability is the enthalpy of lattice formation, not simply an octet around the ions in the gaseous state (page 107).
Watch-out. Lattice enthalpy cannot be calculated directly from attraction and repulsion alone. Factors associated with the crystal geometry have to be included (page 107).
How do you solve a Ionic Bonding Lattice question? A worked example
Pattern: Show why an ionic solid is stable although ionization enthalpy plus electron gain enthalpy is positive, using NCERT's NaCl data. (The dossier has no PYQ pattern; this follows NCERT's own example.)
- 1
Given
Ionization enthalpy of Na(g): 495.8 kJ mol⁻¹. Electron gain enthalpy of Cl(g): −348.7 kJ mol⁻¹. Enthalpy of lattice formation of NaCl(s): −788 kJ mol⁻¹.
- 2
Required
The sum of the first two terms, and how much the lattice formation enthalpy outweighs it.
- 3
Concept
Ionic compounds are stabilised by the energy released when the crystal lattice forms, even if the sum of ionization enthalpy and electron gain enthalpy is positive (NCERT Class 11 Chemistry, Chapter 4, page 106).
- 4
Formula
Sum of the ion-forming terms = ionization enthalpy + electron gain enthalpy. Compare it with the magnitude of the lattice formation enthalpy.
- 5
Substitution
Sum = 495.8 + (−348.7). Net with the lattice term = sum + (−788).
- 6
Calculation
Sum = 147.1 kJ mol⁻¹. Then 147.1 − 788 = −640.9, which rounds to −641 kJ mol⁻¹ because 788 has no decimal places. The lattice term outweighs the positive sum by 641 kJ mol⁻¹. This compares only the three terms NCERT gives. It is not a complete energy balance for forming NaCl from its elements. The number 1 in "one mole" is an exact count.
- 7
Final answer
The sum of the ion-forming terms is +147.1 kJ mol⁻¹, and the lattice formation enthalpy (−788 kJ mol⁻¹) outweighs it by about 641 kJ mol⁻¹, so the crystal is stabilised.
- 8
Common trap
Concluding that a positive sum means the ionic compound cannot form, or that achieving an octet alone explains stability. NCERT says the lattice enthalpy is the measure of stability (page 107).
- 9
Similar NEET-style question
How much energy is needed to separate 2 mol of solid NaCl into gaseous ions? (Answer: 2 × 788 kJ = 1576 kJ ≈ 1.58 × 10³ kJ. The 2 is an exact count, so three significant figures from 788 are kept.)
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Can you answer these Ionic Bonding Lattice MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
What gives a qualitative measure of the stability of an ionic compound?
Show answer and why every option is right or wrong
Answer: B. B is correct. NCERT: “Thus a qualitative measure of the stability of an ionic compound is provided by its enthalpy of lattice formation and not simply by achieving octet of electrons around the ionic species in gaseous state.” (NCERT Class 11 Chemistry, Chapter 4, page 107).
Why A is wrong: A is wrong because NCERT says stability is measured by lattice formation enthalpy and not simply by achieving an octet around the gaseous ions (trap: octet equals stability).
Why C is wrong: C is wrong because ionization enthalpy is only one of the contributions to forming the ions. It does not measure the stability of the compound.
Why D is wrong: D is wrong because electron gain enthalpy is likewise only one contribution. The lattice enthalpy is the measure NCERT gives.
MCQ 2Easy RecallPractice
The lattice enthalpy of an ionic solid is the energy required to:
Show answer and why every option is right or wrong
Answer: D. D is correct. NCERT: “The Lattice Enthalpy of an ionic solid is defined as the energy required to completely separate one mole of a solid ionic compound into gaseous constituent ions.” (NCERT Class 11 Chemistry, Chapter 4, page 107).
Why A is wrong: A is wrong because removing one electron from a gaseous atom is the ionization enthalpy, a different quantity.
Why B is wrong: B is wrong because adding an electron to a gaseous atom is the electron gain enthalpy, a different quantity.
Why C is wrong: C is wrong because forming the solid from its elements is an enthalpy of formation. Lattice enthalpy is about separating the solid into gaseous ions.
MCQ 3Easy RecallPractice
In the crystalline state an ionic compound consists of:
Show answer and why every option is right or wrong
Answer: A. A is correct. NCERT: “Ionic compounds in the crystalline state consist of orderly three-dimensional arrangements of cations and anions held together by coulombic interaction energies.” (NCERT Class 11 Chemistry, Chapter 4, page 106).
Why B is wrong: B is wrong because shared electron pairs describe covalent bonds. In sodium chloride crystals the ions are held by coulombic interactions.
Why C is wrong: C is wrong because the solid is made of ions, not neutral atoms.
Why D is wrong: D is wrong because the arrangement is orderly and three-dimensional, such as the rock salt structure of NaCl.
MCQ 4Direct ApplicationPractice
Ionic bonds form most easily between elements with:
Show answer and why every option is right or wrong
Answer: C. C is correct. NCERT: “Obviously ionic bonds will be formed more easily between elements with comparatively low ionization enthalpies and elements with comparatively high negative value of electron gain enthalpy.” (NCERT Class 11 Chemistry, Chapter 4, page 106).
Why A is wrong: A is wrong because a low negative electron gain enthalpy means the anion forms less easily, so the pair is not favourable.
Why B is wrong: B is wrong because a high ionization enthalpy makes the cation hard to form.
Why D is wrong: D is wrong because both properties are unfavourable here: the cation forms with difficulty and the anion forms with little energy gain.
MCQ 5Direct ApplicationPractice
The lattice enthalpy of NaCl is 788 kJ mol⁻¹. What does this mean?
Show answer and why every option is right or wrong
Answer: D. D is correct. Based on NCERT Class 11 Chemistry, Chapter 4, page 107.
Why A is wrong: A is wrong because it describes forming NaCl from its elements, an enthalpy of formation, not lattice enthalpy (trap: mixing up lattice enthalpy with formation).
Why B is wrong: B is wrong because that is the ionization enthalpy of sodium, 495.8 kJ mol⁻¹ in NCERT's example.
Why C is wrong: C is wrong because that is the electron gain enthalpy of chlorine, −348.7 kJ mol⁻¹ in NCERT's example.
MCQ 6Direct ApplicationPractice
The ionization enthalpy for Na(g) → Na⁺(g) is 495.8 kJ mol⁻¹ and the electron gain enthalpy for Cl(g) + e⁻ → Cl⁻(g) is −348.7 kJ mol⁻¹. What is their sum?
Show answer and why every option is right or wrong
Answer: B. B is correct: 495.8 + (−348.7) = +147.1 kJ mol⁻¹, a positive value that the lattice formation enthalpy more than compensates. Based on NCERT Class 11 Chemistry, Chapter 4, page 106.
Why A is wrong: A is wrong because it adds the magnitudes and then gives the result the wrong sign: −(495.8 + 348.7).
Why C is wrong: C is wrong because it adds 495.8 and 348.7, ignoring that the electron gain enthalpy is negative.
Why D is wrong: D is wrong because the sign is reversed. The larger term is the positive ionization enthalpy, so the sum is positive.
MCQ 7CalculationPractice
Consider these statements. (i) The stability of an ionic compound is judged only by achieving an octet around the gaseous ions. (ii) For NaCl the sum of ionization enthalpy and electron gain enthalpy is positive, yet the crystal is stable. (iii) Lattice enthalpy can be calculated directly from attraction and repulsion alone. (iv) Ionic solids are orderly three-dimensional arrangements of ions. Which are correct?
Show answer and why every option is right or wrong
Answer: C. C is correct. NCERT: “In ionic solids, the sum of the electron gain enthalpy and the ionization enthalpy may be positive but still the crystal structure gets stabilized due to the energy released in the formation of the crystal lattice.” (NCERT Class 11 Chemistry, Chapter 4, page 106). Statement (iv) is also on page 106.
Why A is wrong: A is wrong because (i) is false: stability is measured by lattice enthalpy and not simply by achieving an octet (trap: octet equals stability).
Why B is wrong: B is wrong because (iii) is false: it is not possible to calculate lattice enthalpy directly from attraction and repulsion only, since crystal geometry factors are needed.
Why D is wrong: D is wrong because (iii) is false, even though (iv) is correct.
MCQ 8CalculationPractice
A student estimates the lattice enthalpy of NaCl from the attraction between one Na⁺ and one Cl⁻ only. What is missing?
Show answer and why every option is right or wrong
Answer: A. A is correct. The process involves attraction between opposite charges and repulsion between like charges in a three-dimensional crystal. NCERT: “Factors associated with the crystal geometry have to be included.” (NCERT Class 11 Chemistry, Chapter 4, page 107).
Why B is wrong: B is wrong because the crystal has both attractive and repulsive forces, and direct calculation from forces alone is not possible (trap: one ion pair stands for the lattice).
Why C is wrong: C is wrong because chlorine's ionization enthalpy plays no role in separating NaCl(s) into gaseous Na⁺ and Cl⁻.
Why D is wrong: D is wrong because lattice enthalpy is defined for a solid ionic compound; NaCl's value is 788 kJ mol⁻¹.
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What to remember before solving Ionic Bonding Lattice questions
8 NCERT lines
Obviously ionic bonds will be formed more easily between elements with comparatively low ionization enthalpies and elements with comparatively high negative value of electron gain enthalpy.
-- NCERT Class 11 Chemistry, Ch. 4, p. 106Ionic compounds in the crystalline state consist of orderly three-dimensional arrangements of cations and anions held together by coulombic interaction energies. These compounds crystallise in different crystal structures determined by the size of the ions, their packing arrangements and other factors. The crystal structure of sodium chloride, NaCl (rock salt), for example is shown below.
-- NCERT Class 11 Chemistry, Ch. 4, p. 106In ionic solids, the sum of the electron gain enthalpy and the ionization enthalpy may be positive but still the crystal structure gets stabilized due to the energy released in the formation of the crystal lattice. For example: the ionization enthalpy for Na+(g) formation from Na(g) is 495.8 kJ mol–1 ; while the electron gain enthalpy for the change Cl(g) + e–→ Cl– (g) is, – 348.7 kJ mol–1 only. The sum of the two, 147.1 kJ mol-1 is more than compensated for by the enthalpy of lattice formation of NaCl(s) (–788 kJ mol–1).
-- NCERT Class 11 Chemistry, Ch. 4, p. 106Key Fact
Stability of an ionic compound is measured by lattice enthalpy, not simply by achieving an octet
Thus a qualitative measure of the stability of an ionic compound is provided by its enthalpy of lattice formation and not simply by achieving octet of electrons around the ionic species in gaseous state.
-- NCERT Class 11 Chemistry, Ch. 4, p. 107The Lattice Enthalpy of an ionic solid is defined as the energy required to completely separate one mole of a solid ionic compound into gaseous constituent ions. For example, the lattice enthalpy of NaCl is 788 kJ mol–1. This means that 788 kJ of energy is required to separate one mole of solid NaCl into one mole of Na+ (g) and one mole of Cl– (g) to an infinite distance.
-- NCERT Class 11 Chemistry, Ch. 4, p. 107This process involves both the attractive forces between ions of opposite charges and the repulsive forces between ions of like charge. The solid crystal being three- dimensional; it is not possible to calculate lattice enthalpy directly from the interaction of forces of attraction and repulsion only. Factors associated with the crystal geometry have to be included.
-- NCERT Class 11 Chemistry, Ch. 4, p. 107Definition
Ionic and covalent bonds
Ionic bond: electrostatic attraction between cation and anion (electron transfer). Covalent bond: sharing of electron pair between two atoms. Polar covalent: unequal sharing due to electronegativity difference.
-- NCERT Class 11 Chemistry, Ch. 4, p. 102Ionic Bonding Lattice: NEET previous year questions (PYQs) with answers
18 questions in Chemical Bonding and Molecular Structure
No question in our NEET 2020–2025 set targets this topic directly.
All 18 past-paper questions from Chemical Bonding and Molecular Structure →
More in Chemical Bonding and Molecular Structure: 4 exam traps and mistakes · 2 formulas · 3 question patterns from its other lessons.
Sources
NCERT refs: Class 11 Chemistry Chapter 4, p.106 | Class 11 Chemistry Chapter 4, p.107
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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