Every bond is a tug of war over a shared pair of electrons. Electronegativity measures how hard each atom pulls, and the difference between the two decides whether the bond is covalent, polar or ionic.
Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond towards itself. It increases with nuclear charge and decreases as the atomic radius and shielding increase, so it rises across a period and falls down a group; fluorine (4.0 on the Pauling scale) is the most electronegative element. When two bonded atoms have equal electronegativities the bond is pure covalent. A small difference gives a polar covalent bond with δ+ and δ− ends; a large difference (roughly 2 or more) means the electron transfer is essentially complete and the bonding is ionic.
Pick pairs of atoms in the simulator and watch where the bonding pair sits. Try H–H, then H–Cl, then H–F: the pair slides towards the atom with the higher electronegativity and the bond gains δ+ and δ− ends.
| Factor | Effect on electronegativity | Why |
|---|---|---|
| nuclear charge | higher charge, higher electronegativity | more protons attract the bonding pair more strongly |
| atomic radius | larger atom, lower electronegativity | the bonding pair is further from the nucleus |
| shielding | more inner shells, lower electronegativity | inner electrons screen the nuclear attraction |
Across a period the nuclear charge rises while shielding stays almost the same, so the atoms shrink and pull harder. Down a group each new shell adds distance and shielding, which outweighs the extra protons.
| Period 2 | Li 1.0 | Be 1.5 | B 2.0 | C 2.5 | N 3.0 | O 3.5 | F 4.0 |
|---|---|---|---|---|---|---|---|
| Group 17 | F 4.0 | Cl 3.0 | Br 2.8 | I 2.5 |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Electronegativity is best defined as the power of an atom to:
Which element has the highest electronegativity?
How does electronegativity change down Group 17?
In the bond H–Cl, which statement is correct?
Subtract the two electronegativities. The bigger the difference, the more uneven the sharing, until one atom effectively owns the pair.
Electronegativity is about electrons in a bond. Do not confuse it with electron affinity or ionisation energy, which describe isolated gaseous atoms. And a molecule can contain polar bonds yet be non-polar overall if the dipoles cancel by symmetry, as in CO2; that idea returns in the intermolecular forces lesson.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which bond is the most polar? (Electronegativities: H 2.1, C 2.5, N 3.0, O 3.5, F 4.0)
Why does electronegativity increase across Period 3 from sodium to chlorine?
Which pair of elements is most likely to form an ionic compound? (Electronegativities: Na 0.9, Mg 1.2, C 2.5, S 2.5, Cl 3.0, O 3.5)
Which statement about the C–Cl bond in tetrachloromethane, CCl4, is correct? (C 2.5, Cl 3.0)
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