The concept of ionic and covalent bonds is where NEET tests whether you actually understand why atoms bond, not just that they bond. A common confusion: treating ionic and covalent as a binary switch. They are endpoints on a continuum — most real bonds have partial ionic and partial covalent character.
Ionic bond forms when one atom transfers one or more electrons to another, producing oppositely charged ions held by electrostatic attraction. This typically occurs between elements with a large electronegativity difference (generally > 1.7, though this threshold is approximate). Metals from Groups 1 and 2 with nonmetals from Groups 16 and 17 are classic examples: NaCl, MgO, CaF₂.
Covalent bond forms when two atoms share one or more electron pairs. When the sharing is equal (same atoms or negligible electronegativity difference), the bond is nonpolar covalent: H₂, Cl₂, N₂. When sharing is unequal, the bond is polar covalent: HCl, H₂O.
Key NCERT distinction (Class 11 Chemistry Chapter 4, page 101): a chemical bond is the attractive force that holds two atoms together. The octet rule — atoms tend to achieve eight electrons in their valence shell — drives both bond types, but through different mechanisms: electron transfer (ionic) versus electron sharing (covalent).
Watch-out for NEET: Questions test whether you can identify bond type from electronegativity difference, distinguish lattice structures from molecular structures, and recognise that "ionic" does not automatically mean "solid at room temperature" — it means a lattice of ions with high melting point, electrical conductivity when molten or dissolved, but not when solid.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following best describes an ionic bond?
Answer: D. An ionic bond involves electron transfer from a metal to a nonmetal, creating cations and anions held together by electrostatic force of attraction (NCERT Class 11 Chemistry Chapter 4, page 101).
Why A is wrong: A describes a covalent bond, not ionic. In ionic bonding, electrons are transferred, not shared.
Why B is wrong: B describes metallic bonding or delocalised π-systems, not ionic bonding.
Why C is wrong: C describes covalent bond formation through the valence bond theory approach (orbital overlap), not ionic bonding.
In which of the following compounds is the bond predominantly covalent?
Answer: C. CCl₄ involves carbon sharing electrons with four chlorine atoms. The electronegativity difference between C (2.5) and Cl (3.0) is 0.5, which is well within the covalent range. NaCl, KBr, and CaO all have large electronegativity differences and are predominantly ionic.
Why A is wrong: NaCl has an electronegativity difference of ~2.1 (Na: 0.9, Cl: 3.0), placing it firmly in the ionic category.
Why B is wrong: KBr has an electronegativity difference of ~2.0 (K: 0.8, Br: 2.8), making it predominantly ionic.
Why D is wrong: CaO has an electronegativity difference of ~2.5 (Ca: 1.0, O: 3.5), making it strongly ionic.
Which property is characteristic of ionic compounds?
Answer: B. Ionic compounds form a crystal lattice of ions. Breaking the lattice requires substantial energy (high melting point). In the molten state, ions are free to move and conduct electricity. In the solid state, ions are fixed and cannot conduct (NCERT Class 11 Chemistry Chapter 4).
Why A is wrong: Ionic compounds have high melting points due to strong electrostatic forces in the crystal lattice, not low melting points.
Why C is wrong: Ionic compounds do NOT conduct electricity in the solid state — ions are locked in fixed lattice positions and cannot migrate.
Why D is wrong: Ionic compounds are generally soluble in polar solvents (like water), not nonpolar solvents. 'Like dissolves like' — ionic is polar.
The electronegativity values of atoms X and Y are 0.9 and 3.5 respectively. The bond formed between X and Y is most likely:
Answer: A. The electronegativity difference is 3.5 − 0.9 = 2.6, which is large enough to classify the bond as ionic. As a general guideline, a difference greater than ~1.7 typically indicates ionic character.
Why B is wrong: Polar covalent bonds typically occur with electronegativity differences in the range ~0.4–1.7. A difference of 2.6 exceeds this range, placing the bond in the ionic category.
Why C is wrong: Nonpolar covalent bonds form when the electronegativity difference is zero or negligible (< 0.4). A difference of 2.6 is far too large for nonpolar covalent.
Why D is wrong: Metallic bonding involves delocalisation of electrons in a 'sea' among metal atoms. It does not describe a bond between a low-EN atom and a high-EN atom.
Which of the following molecules contains a nonpolar covalent bond?
Answer: D. Cl₂ consists of two identical chlorine atoms. The electronegativity difference is zero, so the electron pair is shared equally, producing a nonpolar covalent bond.
Why A is wrong: HF has a large electronegativity difference (H: 2.1, F: 4.0, difference = 1.9). The bond is highly polar covalent, bordering on ionic character.
Why B is wrong: H₂O has polar covalent O–H bonds (electronegativity difference ~1.4). The sharing is unequal, with oxygen pulling electron density toward itself.
Why C is wrong: NaF is ionic (electronegativity difference ~3.1). Sodium transfers its valence electron to fluorine.
An ionic compound is dissolved in water and the solution conducts electricity. The same compound in the solid state does not conduct. This is because:
Answer: B. In an ionic solid, ions are held in fixed lattice positions and cannot migrate. When dissolved in water, the crystal lattice breaks apart, releasing free ions that carry charge through the solution (NCERT Class 11 Chemistry Chapter 4).
Why A is wrong: Water does not destroy ions — it solvates (surrounds) them through ion-dipole interactions, keeping them intact as charged species.
Why C is wrong: Dissolving an ionic compound in water is a physical dissociation process (ion-dipole interaction), not covalent bond formation between solute and solvent.
Why D is wrong: In ionic conduction (electrolytic), charge is carried by migrating ions, not by free electrons. Electron flow describes metallic conduction.
A student claims that since MgO and NaCl are both ionic, they should have nearly identical melting points. What is wrong with this reasoning?
Answer: C. Lattice energy is proportional to the product of ionic charges and inversely proportional to the interionic distance (Coulomb's law applied to ionic lattices). MgO has +2 and −2 charges versus +1 and −1 in NaCl, and smaller ionic radii. Both factors give MgO a much higher lattice energy (3850 kJ/mol vs 786 kJ/mol) and correspondingly higher melting point (2852 °C vs 801 °C).
Why A is wrong: MgO is strongly ionic — Mg has electronegativity 1.2 and O has 3.5, giving a difference of 2.3. The claim that MgO is covalent is incorrect.
Why B is wrong: Both NaCl and MgO adopt the same rock-salt (face-centred cubic) crystal structure. The difference in melting points arises from charge magnitude and ionic size, not crystal structure type.
Why D is wrong: Melting point of ionic compounds depends on lattice energy, which is governed by ionic charge and size, not molecular mass. CO₂ (mass 44) is a gas at room temperature while NaCl (mass 58.5) melts at 801 °C — mass is irrelevant.
Consider the series: NaF, NaCl, NaBr, NaI. As we go from NaF to NaI, the ionic character of the bond:
Answer: A. Electronegativity decreases down Group 17: F (4.0) > Cl (3.0) > Br (2.8) > I (2.5). Since Na has a fixed electronegativity of 0.9, the electronegativity difference decreases from NaF (3.1) to NaI (1.6). A smaller electronegativity difference means less ionic character, so ionic character decreases from NaF to NaI.
Why B is wrong: Iodine is LESS electronegative than fluorine (I: 2.5 vs F: 4.0). Electronegativity decreases down the group as atomic size increases and nuclear pull on bonding electrons weakens.
Why C is wrong: While all four are classified as ionic compounds, the degree of ionic character is not identical. Fajan's rules and electronegativity differences both predict variation in ionic character within a series.
Why D is wrong: There is no reversal in this trend. Electronegativity decreases monotonically down Group 17, so the ionic character decreases monotonically from NaF to NaI.
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Given
A compound is formed between element A (electronegativity = 1.0) and element B (electronegativity = 3.0). The compound has a melting point of 801 °C, does not conduct electricity as a solid, but conducts when dissolved in water.
Required
Identify the bond type and justify using both electronegativity data and observed properties.
Concept
Bond type can be predicted from the electronegativity difference between bonded atoms. A large difference (generally > 1.7) indicates ionic bonding. Ionic compounds exhibit: high melting points (strong lattice), no solid-state conductivity (fixed ions), and conductivity in solution (free ions).
Formula
Electronegativity difference = |EN_B − EN_A|
Substitution
ΔEN = |3.0 − 1.0| = 2.0
Calculation
ΔEN = 2.0, which exceeds the ~1.7 threshold, indicating predominantly ionic bonding.
Cross-check with properties:• High melting point (801 °C) ✓ — consistent with ionic lattice• No solid-state conductivity ✓ — ions fixed in lattice• Conductivity in aqueous solution ✓ — free ions in water
All three observations confirm ionic bonding.
Final answer
The bond between A and B is ionic. The electronegativity difference of 2.0 and all three observed properties (high MP, no solid conductivity, solution conductivity) are consistent with ionic bonding.
Common trap
Concluding "ionic" from electronegativity difference alone without verifying against properties, or vice versa. NEET questions sometimes give a compound with a borderline ΔEN (~1.7) where properties become the deciding factor. Always use both criteria when available.
Similar NEET-style question
A compound AB₂ has ΔEN = 1.8, melts at 714 °C, and conducts electricity when molten but not when solid. A student argues the bond is "polar covalent because ΔEN is close to 1.7." Is the student correct? Justify.
(Answer: No — the properties (high MP, conduction only when molten) are definitive markers of ionic bonding. The ΔEN of 1.8 supports ionic character. Borderline ΔEN values require property-based confirmation, which here clearly points to ionic.)
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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. 102More in Chemical Bonding and Molecular Structure: 4 exam traps and mistakes · 2 formulas · 3 question patterns from its other lessons.
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