Ionic vs Covalent Bonding — A Continuum, Not Two Boxes
School chemistry starts by sorting bonds into two bins: metal plus non-metal is ionic, non-metal plus non-metal is covalent. It is a useful first sort, and it fails on some of the most commonly asked compounds — AlCl₃, BeCl₂, AgI. The honest picture is that pure ionic and pure covalent are the two ends of one continuous scale, and almost every real bond sits somewhere in between. This article shows how to place a bond on that scale and how to defend the placement in an exam answer.
The two extremes
- Pure covalent (non-polar): the electron pair is shared equally. This happens only when the two atoms are identical — H₂, Cl₂, O₂.
- Pure ionic: one atom takes the electron completely, producing separate ions. This is an idealisation; no real bond reaches 100% ionic character, because every cation pulls at least slightly on the anion's electron cloud.
Everything else is polar covalent: shared, but unequally. The bonding electrons spend more time near the more electronegative atom, which acquires a partial negative charge δ− while its partner gets δ+.
The first tool — electronegativity difference
Electronegativity (χ) is an atom's pull on a shared electron pair. On the Pauling scale, fluorine is highest at 3.98 and caesium among the lowest at 0.79. The bigger the difference Δχ between two bonded atoms, the more ionic the bond.
| Bond | χ values (Pauling) | Δχ | Usual description |
|---|---|---|---|
| Cl–Cl | 3.16, 3.16 | 0.00 | Pure covalent |
| C–H | 2.55, 2.20 | 0.35 | Very slightly polar |
| H–Cl | 2.20, 3.16 | 0.96 | Polar covalent |
| H–O | 2.20, 3.44 | 1.24 | Strongly polar covalent |
| Al–Cl | 1.61, 3.16 | 1.55 | Covalent in practice — see below |
| Na–Cl | 0.93, 3.16 | 2.23 | Largely ionic |
| K–F | 0.82, 3.98 | 3.16 | About as ionic as bonds get |
You will see a rule quoted as "Δχ greater than 1.7 means ionic". Treat it as a rough guideline only. It comes from Pauling's estimate that Δχ ≈ 1.7 corresponds to about 50% ionic character; different textbooks quote the threshold as 1.7, 1.8 or 2.0, and none of them is a law of nature. AlCl₃ has Δχ = 1.55 yet behaves as a covalent compound — it sublimes below 200 °C and exists as Al₂Cl₆ dimers in the vapour, which no ionic lattice would do.
Worked example — percent ionic character of HCl, by two routes
Route 1 — Pauling's empirical equation.
% ionic character = 100 × [1 − e−0.25(Δχ)²]
For H–Cl, Δχ = 3.16 − 2.20 = 0.96
(0.96)² = 0.9216
0.25 × 0.9216 = 0.2304
e−0.2304 = 0.7942
1 − 0.7942 = 0.2058 → 20.6% ionic character
Route 2 — from the measured dipole moment.
% ionic character = (μobserved / μ100% ionic) × 100, where the fully ionic value is one full electronic charge separated by the bond length. In debye units, μionic = 4.803 × d, with d in ångström.
For HCl, d = 127.4 pm = 1.274 Å and μobserved = 1.08 D
μionic = 4.803 × 1.274 = 6.12 D
% ionic = (1.08 / 6.12) × 100 = 17.6%
Two independent routes give 20.6% and 17.6%. They agree on the chemistry — the H–Cl bond is roughly one-fifth ionic and four-fifths covalent — while differing in the second figure, because Pauling's equation is an empirical fit, not an exact relation. Quoting either value is fine provided you say which method you used.
The second tool — Fajans' rules (covalent character in an "ionic" compound)
Electronegativity looks at the bond from the covalent end. Fajans' rules look at it from the ionic end: given two ions, how much does the cation distort the anion's electron cloud back toward itself? The more distortion (polarisation), the more covalent the bond.
| Factor | More covalent character when… | Illustration |
|---|---|---|
| Cation size | The cation is small | LiCl (m.p. about 605 °C) melts far below NaCl (801 °C) |
| Cation charge | The charge is high | AlCl₃ is molecular; NaCl is a lattice |
| Anion size | The anion is large and easily polarised | AgI is much more covalent than AgF |
| Anion charge | The anion charge is high | Oxides and nitrides show more covalency than the matching halides |
| Cation electron configuration | The cation has a pseudo-noble-gas (18-electron) shell | Ag⁺ (4d¹⁰) polarises far more strongly than Na⁺ of similar size |
Small size plus high charge is summarised as high polarising power, roughly charge divided by radius. Be²⁺ and Al³⁺ are the classic high-polarising-power ions, which is exactly why BeCl₂ and AlCl₃ refuse to behave like ionic salts.
What the properties tell you
| Property | Mostly ionic | Polar covalent (molecular) | Giant covalent |
|---|---|---|---|
| Example | NaCl | HCl | SiO₂ (quartz) |
| Melting point | High — NaCl melts at 801 °C | Low — HCl boils at about −85 °C | Very high — about 1700 °C |
| Conducts when molten or in solution | Yes, ions are free | Only if it ionises in water | No |
| Conducts as a solid | No, ions are fixed | No | No (graphite is the famous exception) |
| Solubility | Often soluble in water | Depends on polarity | Insoluble |
| Structure | 3-D lattice, no molecules | Discrete molecules | Continuous network |
The giant-covalent column is the one students forget. "Covalent means low melting point" is only true for molecular covalent substances, where weak forces between molecules are what actually melt. In diamond and quartz the covalent bonds themselves must break, so the melting point is enormous.
Common mistakes that cost marks
- Treating the 1.7 rule as a law. It is a guideline with famous exceptions. Support your classification with a property — melting point, conductivity in the molten state, volatility — not just a number.
- "Metal + non-metal is always ionic." AlCl₃, BeCl₂ and SnCl₄ are all metal–non-metal and all essentially covalent.
- "Covalent means low melting." Not for diamond, silicon carbide or quartz.
- Confusing a polar bond with a polar molecule. CO₂ has two strongly polar C=O bonds, but the molecule is linear and the two bond dipoles cancel exactly, so the molecule is non-polar. Water is bent, so its dipoles do not cancel and it is polar.
- Saying ionic solids conduct electricity. Solid NaCl does not — the ions are locked in the lattice. It conducts only when molten or dissolved.
- Ignoring the direction of Fajans' rules. They explain covalent character appearing in an ionic compound. Using them to argue the opposite is a common slip in written answers.
- Assuming higher Δχ always means a stronger bond. Ionic character and bond strength are different questions; strength depends on lattice energy or bond enthalpy, not on Δχ alone.
Where this appears in exams
| Exam | Typical question |
|---|---|
| CBSE / ICSE Class 11 | Compare ionic and covalent compounds; explain why AlCl₃ is covalent using Fajans' rules |
| NEET / JEE | Order compounds by covalent character; percent ionic character from dipole moment |
| IIT-JAM / CUET-PG | Polarising power, lattice energy trends, anomalous behaviour of Li and Be |
| GATE / CSIR-NET | Hard–soft acid–base reasoning, solid-state structure vs bond type |
Look up the electronegativity values yourself. Every Δχ in this article came from the periodic table. The interactive table gives electronegativity, atomic radius, ionisation energy and oxidation states for all 118 elements, so you can build any comparison a question asks for.
Open the Interactive Periodic Table →Chemical bonding is the chapter that decides how the rest of inorganic chemistry feels. ABC Chemistry teaches Class 11–12 chemistry at the Gurugram centre and online across India — details at abcchemistry.in. For one-to-one help at home in Delhi, Noida or Gurgaon, see delhihometutor.com.