Effective Nuclear Charge
The net positive charge actually experienced by a particular electron after allowing for shielding by other electrons.
Formula / equation
Z_eff = Z − σ
Unit
Dimensionless
In depth
Estimated using Slater's rules, effective nuclear charge rises steadily across a period as protons are added while shielding barely changes, and stays roughly constant down a group. This single quantity explains the periodic trends in atomic radius, ionisation energy, electron affinity and electronegativity.
Examples in the real world
Across period 2, Z_eff on a valence electron climbs from about 1.3 in lithium to 4.5 in fluorine.