Shielding Effect
The reduction in the nuclear attraction felt by an outer electron caused by repulsion from electrons in inner shells.
Formula / equation
Z_eff = Z − σ
Unit
Dimensionless
In depth
Inner electrons partially cancel the nuclear charge, so an outer electron experiences an effective rather than full charge. Shielding is most effective from s electrons, which penetrate close to the nucleus, and least from f electrons, whose poor shielding causes the lanthanide contraction.
Examples in the real world
Sodium's 3s electron feels an effective charge near +2.2 rather than the full +11 of the nucleus.