Electronegativity & the Pauling Scale
What the number means, the scale for the elements you meet most, and how a difference becomes nonpolar, polar or ionic, with the cutoffs and where they mislead.
Why it matters
is the one number that decides how a bond shares its electrons: evenly, unevenly, or not at all. Every question about polarity, partial charges and ionic versus covalent starts with it.
When you use it
Deciding whether a bond is , which end is negative, and whether a compound is better drawn as shared pairs or as ions.
Where it leads
Molecular polarity, , oxidation states, and reading a Lewis structure for where the electrons really sit.
What the number is
The Pauling scale gives each element a pure number, with no units, for how strongly an atom of it pulls on the electrons it shares in a bond. It is a comparison, not a measurement of one atom alone: it only means something next to another element's number. On this scale fluorine is highest at 3.98 and francium is lowest at 0.70, so every value sits between those two.
The scale for the elements you meet most
- Cs0.79
- K0.82
- Na0.93
- Li0.98
- Ca1.00
- Mg1.31
- Al1.61
- Si1.90
- P2.19
- H2.20
- C2.55
- S2.58
- I2.66
- Br2.96
- N3.04
- Cl3.16
- O3.44
- F3.98
Hydrogen (2.20) sits in the middle of the nonmetals, close to carbon (2.55) and phosphorus (2.19). That is why C-H bonds are nearly nonpolar and why a hydrogen bonded to oxygen or nitrogen is the positive end.
From a difference to a kind of bond
Subtract the two numbers and ignore the sign. The size of that difference, written ΔEN, says how unevenly the pair is shared. These cutoffs are conventions laid over a smooth continuum, not boundaries in nature.
Below 0.4
Nonpolar covalent. Shared about equally.
0.4 up to about 1.7
Polar covalent. Shared unevenly; the pair sits closer to one atom.
Above about 1.7
Ionic, when a metal is involved. The electrons are treated as transferred.
| Bond | Values | ΔEN | Treated as | Negative end |
|---|---|---|---|---|
| H-H | 2.20, 2.20 | 0.00 | Nonpolar covalent | none |
| C-H | 2.55, 2.20 | 0.35 | Nonpolar covalent | C |
| N-H | 3.04, 2.20 | 0.84 | Polar covalent | N |
| O-H | 3.44, 2.20 | 1.24 | Polar covalent | O |
| H-F | 2.20, 3.98 | 1.78 | Polar covalent | F |
| Na-Cl | 0.93, 3.16 | 2.23 | Drawn as ions | Cl |
| Mg-O | 1.31, 3.44 | 2.13 | Drawn as ions | O |
Where the cutoffs mislead
Which end is negative
The more electronegative atom holds the shared pair more of the time, so it carries the partial negative charge (δ-) and the other end the partial positive (δ+). In O-H, oxygen (3.44) is the negative end and hydrogen (2.20) the positive. The charge is partial: the bond is not broken, the electrons are only shared unequally.
Why it moves the way it does
Across a period, nuclear charge rises while the outer shell stays the same distance away, so the nucleus pulls harder on a shared pair. Down a group, an added shell puts the outer electrons farther away and behind more shielding, so the pull weakens even though the nuclear charge is larger.
A common mix-up
The full set of trends, with the numbers for each, is on the Periodic Trends sheet.
Elements with no value
The Pauling scale is built from the energies of the bonds an element forms. Helium, neon, argon, radon form essentially none, so they have no value; a blank is the honest answer, never zero. The superheavy elements from lawrencium onward have never been made in amounts that allow it to be measured. When a bond involves one of these, the electronegativity rule is silent.
Practice