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Students learn that NaCl is ionic and HCl is covalent, then meet a question asking which of AlCl₃ and NaCl is more covalent, and the two-box picture fails. Real bonds sit on a scale between the two. Fajans' rules tell you which way a compound leans. This post sets out NCERT's version of the rules, explains the idea behind them and works four comparisons with data.
Why do ionic compounds have covalent character?
The NCERT Class 11 chapter Chemical Bonding and Molecular Structure says that in reality no bond or compound is completely covalent or completely ionic. Just as covalent bonds have some ionic character, ionic bonds have partial covalent character, which Fajans described in terms of a set of rules. Both the JEE (Main) 2026 syllabus and the NEET (UG)-2026 syllabus list "Fajan's rule" under covalent bonding.
The idea is polarisation. NCERT explains that the cation pulls the anion's electron cloud towards itself, building up electron density between the two nuclei, which is exactly what happens in a covalent bond. So the more the cation can distort the anion, and the more easily the anion is distorted, the more covalent the bond.
What are Fajans' rules?
NCERT lists three:
The smaller the cation and the larger the anion, the greater the covalent character.
The greater the charge on the cation, the greater the covalent character.
For cations of the same size and charge, one with the configuration (n−1)dⁿns⁰, typical of transition metals, is more polarising than one with a noble gas configuration ns²np⁶, typical of alkali and alkaline earth metal cations.
A small, highly charged cation packs its charge into a small space, so it has high polarising power. A large anion holds its outer electrons loosely, so it is easily polarised.
Worked example 1: NaCl, MgCl₂ and AlCl₃
Same anion, Cl⁻. Going from Na⁺ to Mg²⁺ to Al³⁺, the cation charge rises from +1 to +3 and the ion gets smaller, so rules 1 and 2 both point the same way:
Covalent character: NaCl < MgCl₂ < AlCl₃.
Physical data fit this. According to PubChem, sodium chloride melts at 800.7 °C, magnesium chloride at 714 °C, and anhydrous aluminium chloride sublimes readily at 178 °C. A strongly ionic lattice needs a lot of energy to break; a compound with strong covalent character can behave more like a molecular substance.
Worked example 2: the lithium halides
Same cation, Li⁺. Going from F⁻ to Cl⁻ to Br⁻ to I⁻, the anion gets larger and its outer electrons are held more loosely, so by rule 1:
Covalent character: LiF < LiCl < LiBr < LiI.
Use rule 1 for the cation too: among LiCl, NaCl and KCl, Li⁺ is the smallest cation, so LiCl has the most covalent character.
Worked example 3: NaCl and CuCl
Both cations carry +1. Na⁺ has the noble gas configuration 2s²2p⁶. Cu⁺ has the configuration [Ar]3d¹⁰, a d¹⁰ shell of the (n−1)dⁿns⁰ type. By NCERT's third rule, Cu⁺ is more polarising, so CuCl has more covalent character than NaCl.
Worked example 4: when two factors pull in different directions
Compare MgCl₂ with NaI. MgCl₂ has the more highly charged cation, which favours covalent character, but NaI has the larger anion, which also favours it. Rules 1 and 2 now pull in opposite directions, and the simple rules no longer give a confident answer. In an exam, the question will usually be set so that one factor clearly dominates or both point the same way. If they conflict, say so and explain which effect you expect to win and why.
Which mistakes cost marks?
Treating "covalent character" as "covalent compound". NaCl has some covalent character but is still an ionic solid.
Mixing up which ion does what. The cation polarises; the anion is polarised. A large cation is weakly polarising.
Using electronegativity difference alone. It is a different, rougher measure. Fajans' rules ask about ion size, charge and configuration.
Over-reading melting points. Melting point depends on structure as well as bonding, so use data as supporting evidence, not proof.
Applying rule 3 when sizes differ a lot. NCERT states it for cations of the same size and charge.
A practice routine
Write the three rules from memory with one example each.
Rank five sets: the sodium halides; the chlorides of Na, Mg and Al; LiCl, NaCl, KCl; NaCl and CuCl; and one set from your textbook exercises.
For each ranking, name the rule you used in one line.
Link it to lattice energy: read our post on lattice enthalpy and the Born-Haber cycle and note how the same ion sizes and charges appear there.
If polarisation still feels abstract, the Fermi Concept Library has a lesson for every concept in physics, chemistry, biology, mathematics and computing, each placed in a map of related ideas.
Before your next chemistry class, rank the four sodium halides by covalent character and write one sentence of reasoning for each step.
