Effective nuclear charge `(Z_(eff))` is the net attractive force on electrons under consideration and is equal to:
`Z_(eff) = Z - sigma` (nuclear charge - screening constant). `Z_(eff)` or `sigma` is calculated by Slater's formula, as given.
If one electrons is present in the outermst orbit, there will be no screening in that orbital. Each electrons contribute, `0.35` (total electrons minus one electron) present in the outermost shell.
A contribution of `0.85` for each electrons is taken in the `(n - 1)th` shell.
The screening constant `(sigma)` for `4s` electron of `Mn (Z = 25)` will be