Electronegativity
Definition
A measure of how strongly an atom pulls shared electrons toward itself in a bond. Fluorine pulls hardest; metals barely pull at all.
Symbol: · Unit: Pauling units (dimensionless)
Technical definition
Electronegativity is the tendency of an atom to attract shared electron density in a chemical bond, quantified on the Pauling scale where fluorine is 3.98, the highest. It increases across a period and decreases down a group, and differences in electronegativity predict bond polarity.
Example
In H-F, fluorine (3.98) pulls the shared electrons much harder than hydrogen (2.20), making the bond strongly polar.
Electronegativity is not electron affinity. Electronegativity describes attraction for shared electrons within a bond, while electron affinity is the energy change when an isolated atom gains an electron.