Electronegativity
The relative tendency of an atom in a covalent bond to attract the shared pair of electrons towards itself.
Formula / equation
ΔEN determines bond polarity
Unit
Dimensionless
In depth
Pauling's scale runs from 0.79 for caesium to 3.98 for fluorine. Unlike ionisation energy it is a property of a bonded atom, not an isolated one. A difference above roughly 1.7 gives a predominantly ionic bond, 0.4–1.7 a polar covalent bond, and below 0.4 a nearly non-polar one.
Examples in the real world
In H–F the difference of 1.78 makes the bond strongly polar; in H–H it is zero and the bond is non-polar.