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Types of chemical bonds

Ionic, covalent, metallic — they sound like three separate boxes, but bonding is really one continuum set by how evenly two atoms share electrons. Learn to read the electronegativity difference and the boundaries stop tricking you.

§1

Bonding is a spectrum.

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The character of a bond is decided by how evenly the bonding electrons are shared, and that is governed by the atoms' electronegativity difference (ΔEN). It is a continuum, not three sealed categories.

At ΔEN ≈ 0 the sharing is even: a nonpolar covalent bond (Cl₂, C–H). At a moderate ΔEN the more electronegative atom pulls harder, giving a polar covalent bond with a permanent dipole (H–Cl). At a large ΔEN the electron is essentially transferred: an ionic bond (NaCl).

Metals bond a fourth way that is off this axis entirely: a lattice of cations sharing a delocalized sea of electrons. So four outcomes, but the covalent-to-ionic part is a smooth slide, not a set of walls.

UNIT 2 TOPIC 2.1 • TYPES OF CHEMICAL BONDS BOND SPECTRUM Electronegativity difference (ΔEN) is a guide, not the only test. ΔEN ≈ 0 increasing ΔEN large ΔEN NONPOLAR COVALENT EXAMPLE: Cl₂ and C–H electrons shared nearly evenly; no permanent bond dipole Cl Cl symmetric electron cloud C–H bond is effectively nonpolar POLAR COVALENT EXAMPLE: HCl unequal sharing; the more electronegative atom carries δ− H Cl δ+ δ− lopsided electron cloud bond dipole points toward Cl IONIC EXAMPLE: NaCl electron transfer; separated ions attract strongly Na Cl [Na]⁺ [Cl]⁻ separated ions, strong attraction NO HARD CUTOFF ΔEN supports a claim, but element types and measured properties decide which bonding model applies. metal + nonmetal → often ionic BIG IDEA Greater ΔEN means a more uneven electron distribution and a larger bond dipole. more uneven as ΔEN increases TAKEAWAY Bonds are a continuum. ΔEN, element types, and measured properties together support the bonding claim. AP Chemistry · Unit 2 · Compound Structure & Properties
Fig. 2.1.1 Bonding as a spectrum. As ΔEN grows, electron sharing slides from even (nonpolar covalent) to lopsided (polar covalent) to essentially transferred (ionic). Metallic bonding is a separate case: a shared electron sea.
§2

Classifying a bond.

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Do not guess from the element names. Read the electronegativity difference and place the bond.

  1. Find each atom's electronegativity. Use the periodic trend or a given table. Electronegativity rises up and to the right.
  2. Compute ΔEN. Subtract the smaller from the larger. This single number places the bond on the spectrum.
  3. Place it. Roughly: ΔEN under ~0.5 is nonpolar covalent, ~0.5 to ~1.7 is polar covalent, above ~1.7 is ionic. The cutoffs are soft, so treat borderline cases as borderline.
  4. Check for a metal-metal bond. Two metals bond metallically (electron sea), not by ΔEN. That case is off the covalent-ionic axis.
§3

The pieces you'll meet.

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The whole topic runs on one number and what it means.

ΔEN
Electronegativity difference
How unequally two atoms pull the bonding electrons. Sets the bond type.
nonpolar
Nonpolar covalent
ΔEN ≈ 0; electrons shared evenly, no permanent dipole.
polar
Polar covalent
Moderate ΔEN; unequal sharing gives a bond dipole.
ionic
Ionic
Large ΔEN; electron essentially transferred to form ions.
metallic
Metallic
Cations sharing a delocalized electron sea. Off the ΔEN axis.
dipole
Bond dipole
The separation of charge in a polar bond, pointing toward the more electronegative atom.
§4

Worked example: classify three bonds.

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Question. Classify the bonds in Cl₂, HCl, and NaCl. (EN: Cl 3.0, H 2.1, Na 0.9.)

Cl₂. Both atoms are chlorine, ΔEN = 0. Even sharing → nonpolar covalent.

HCl. ΔEN = 3.0 − 2.1 = 0.9. Moderate, so chlorine pulls harder → polar covalent, with the dipole pointing toward Cl.

NaCl. ΔEN = 3.0 − 0.9 = 2.1. Large, so the electron is essentially handed over → ionic. Same two-atom recipe, three different points on one spectrum.

§5

Mistakes that cost real points.

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Pitfall · 01

"Ionic bonding is a clean, all-or-nothing transfer of electrons."

Even 'ionic' bonds keep some electron sharing; the transfer is never perfectly complete, and there is no sharp wall between very-polar covalent and ionic. The ΔEN ≈ 1.7 cutoff is a convention, not a physical cliff. Treating ionic as total transfer misreads borderline compounds.

Fix. See ionic as the far end of a continuous scale of sharing, not a separate mechanism. Near the boundary, expect partial-ionic, partial-covalent character.

Pitfall · 02

"You can tell the bond type just from the elements' labels (metal + nonmetal = ionic)."

The metal-plus-nonmetal shortcut often works but is not the rule; the actual test is ΔEN. Some metal-nonmetal bonds are quite covalent in character. Judging from element type alone will misclassify the borderline cases.

Fix. Compute ΔEN and place the bond on the spectrum. Use element type as a hint, but let the electronegativity difference decide.

Pitfall · 03

"Any nonzero ΔEN makes a bond meaningfully polar."

A tiny ΔEN (like a C–H bond, ΔEN ≈ 0.4) is treated as essentially nonpolar. A difference has to be large enough to matter; a sliver of inequality does not make a strongly polar bond.

Fix. Compare ΔEN against the ranges. Small differences (under ~0.5) count as nonpolar covalent, not polar.

Pitfall · 04

"Metallic bonding is basically nonpolar covalent bonding."

Metallic bonding is not covalent at all. Instead of a shared pair between two atoms, a metal has electrons delocalized across the whole lattice. That is why metals conduct and bend, which nonpolar covalent substances do not.

Fix. Keep metallic bonding as its own category: a sea of delocalized electrons shared by all the cations, not a two-atom shared pair.

§6

Skill Check.

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