Shielding constant ($S$)

The screening parameter $S$ in $Z_{\mathrm{eff}}=Z-S$. Different atomic orbitals can have different shielding parameters.

An empirical orbital-screening prescription. Other electrons in an $ns,np$ group count $0.35$ each; electrons in shell $n-1$ count $0.85$, deeper electrons $1.00$, and outer electrons zero. For $1s$, the partner contributes $0.30$. d and f groups use separate rules.^[ref]