Calculate the `Z_(eff)` at the periphery of the chromium atom.

Explanation

Imagine an extra electron in the outermost filled orbital (4s) of the chromium atom.
Electronic configuration of chromium ion (Cr=24+1 = 25) is `(1s^2) (2s^2 2p^6) (3s^2 3p^6) (3d^8) (4s^2)`
The screening constant for the extra electron electron=`1xx0.35+(8+5)xx0.85+1.0xx10=21.40`.
Therefore, `Z_(eff)` at the periphery of the chromium atom ` = Z - sigma = 24-21.40 -2.60`