The formula
- Bond character from the electronegativity difference
ΔEN = |EN(A) - EN(B)|- Classification bands
ΔEN < 0.4 pure covalent; 0.4 to 1.8 polar covalent; > 1.8 ionic- Bond order and what it does
single < double < triple: stronger and shorter as bond order rises- Shared electrons per bond
single 2 e⁻, double 4 e⁻, triple 6 e⁻
What the symbols mean
| Symbol | Meaning | Unit |
|---|---|---|
ΔEN | Absolute difference in electronegativity between the bonded atoms | dimensionless (Pauling scale) |
EN | Electronegativity, an atom's pull on shared electrons; fluorine is highest at 4.0 | dimensionless, 0 to 4 on the Pauling scale |
bond order | Number of shared electron pairs between the two atoms | 1, 2 or 3 |
bond energy | Energy needed to break one mole of that bond in the gas phase | kJ/mol |
When it applies
- Both atoms are nonmetals or metalloids. A metal with a nonmetal usually transfers electrons outright instead of sharing them.
- The bands are guidelines rather than sharp lines, so a difference near 1.8 describes a bond with substantial character of both kinds.
- ΔEN classifies one bond. Whether the whole molecule is polar also depends on its shape, since symmetric arrangements can cancel the individual bond dipoles.
- Bond energy and bond length go together within a pair of atoms: a triple bond is both stronger and shorter than the double bond between the same two elements.
Worked example
Problem. Classify the bonding in H₂, in HCl and in NaCl using electronegativity differences, and say which molecule carries a bond dipole.
- H₂ joins two identical atoms, so ΔEN = 0. With a difference below 0.4 the bond is pure covalent and the pair is shared equally.
- For H-Cl the difference is 0.9. That sits inside the 0.4 to 1.8 band, so the bond is polar covalent.
- Decide the direction: chlorine is the more electronegative atom, so it carries the partial negative charge and hydrogen the partial positive.
- For Na-Cl the difference is 2.1, above 1.8, so the electrons are transferred rather than shared and the compound is ionic.
- Only HCl among the three has a bond dipole, and because the molecule has just two atoms that bond dipole makes the whole molecule polar.
Answer. H₂ is pure covalent, HCl is polar covalent and carries the bond dipole, and NaCl is ionic.
Common mistakes
- Treating a polar covalent bond as ionic. Sharing unevenly is not the same as transferring, and hydrogen chloride is a molecule rather than a salt.
- Concluding a molecule is polar because it holds polar bonds. Carbon dioxide has two strongly polar bonds whose dipoles cancel, leaving the molecule nonpolar.
- Subtracting electronegativities in the wrong order and keeping the negative sign. The classification uses the absolute difference.
- Assuming a longer bond is stronger. Within the same pair of atoms, bond strength rises as bond length falls.
Related formulas
- Electron configuration:
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p - Lewis structure:
1 count valence electrons, 2 connect atoms with single bonds, 3 complete octets on outer atoms, 4 put leftovers on the central atom, 5 form multiple bonds if the central atom is short