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Every chemical bond is a tug-of-war over electrons. Electronegativity measures how hard each atom pulls. When two atoms pull equally, the electrons are shared evenly; when one pulls much harder, it takes the electrons almost entirely, and the bond becomes ionic. Knowing where an element sits on the periodic table tells you roughly how strong its pull is.
The definition
Electronegativity is the ability of an atom in a molecule to attract the shared electrons of a covalent bond towards itself.
The key words are in a molecule. Unlike ionization energy or electron affinity, it isn’t a property of an isolated atom that you can measure directly. It’s a derived, relative scale.
The Pauling scale
The most widely used scale was devised by Linus Pauling in 1932, based on bond energies: bonds between atoms of different electronegativity turned out to be stronger than expected, and the extra strength could be used to calculate the difference. The scale runs from about 0.7 to 3.98, with no units.
Other scales exist — Mulliken (from ionization energy and electron affinity), Allred–Rochow (from effective nuclear charge and size), Allen (from average electron energies) — and they give slightly different numbers, but the same overall trends. Our element data uses Pauling values.
The trend
- Across a period → increases. Atoms get smaller and effective nuclear charge rises, so bonding electrons are held closer to a more strongly attracting nucleus.
- Down a group → decreases. Atoms get bigger; bonding electrons are further from the nucleus and better shielded.
So electronegativity climbs diagonally towards the top right of the table:
| Period 2 | Li | Be | B | C | N | O | F |
|---|---|---|---|---|---|---|---|
| Pauling | 0.98 | 1.57 | 2.04 | 2.55 | 3.04 | 3.44 | 3.98 |
| Group 17 | F | Cl | Br | I |
|---|---|---|---|---|
| Pauling | 3.98 | 3.16 | 2.96 | 2.66 |
Fluorine is the most electronegative element; cesium and francium are the least (0.79 and about 0.7). See the most electronegative element. Noble gases are usually left out, because helium, neon and argon form no stable compounds; krypton and xenon, which do, have assigned values.
Using electronegativity differences
The difference in electronegativity (ΔEN) between two bonded atoms predicts the bond type:
| ΔEN (Pauling) | Bond type | Examples |
|---|---|---|
| 0 – ~0.4 | Nonpolar covalent | H–H (0), C–H (0.35) |
| ~0.4 – ~1.7 | Polar covalent | O–H (1.24), C–O (0.89), H–Cl (0.96) |
| above ~1.7 | Mostly ionic | Na–Cl (2.23), K–F (3.16) |
These cut-offs are guidelines. Bonding is really a continuum from perfectly shared to completely transferred electrons, and different textbooks draw the lines slightly differently (1.7, 1.8 or 2.0 are all common for the ionic boundary). See ionic vs. covalent bonds and polar vs. nonpolar molecules.
What electronegativity explains
- Which end of a bond is negative. In water, oxygen (3.44) pulls electrons from hydrogen (2.20), so oxygen is δ− and hydrogen δ+ — the origin of hydrogen bonding.
- Oxidation numbers. They’re assigned by giving all shared electrons to the more electronegative atom. See how to find oxidation numbers.
- Which element is named first in covalent compounds (the less electronegative one).
- Reactivity patterns in organic chemistry — electronegative atoms make neighbouring carbons slightly positive and attractive to electron-rich reactants.
The diagonal relationship
Because electronegativity increases to the right but decreases downwards, elements diagonally adjacent (like lithium and magnesium, or beryllium and aluminium, or boron and silicon) have similar electronegativities and show surprisingly similar chemistry. This is called the diagonal relationship.
Quick answers
Does electronegativity have units? No. It’s a relative scale.
Why is fluorine’s value 3.98 and not 4.0? Pauling originally used 4.0; 3.98 is a refined value from later data.
Is electronegativity the same as electron affinity? Related but different: electron affinity is a measured energy for an isolated atom; electronegativity describes an atom’s pull within a bond.
See it
The electronegativity trend map colours the whole table by Pauling value.
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