How this calculator works
Choose an electron in an occupied subshell. Group ns and np together. Other electrons in that group shield by 0.35 each, except 1s where the factor is 0.30. For an ns or np electron, electrons in shell n-1 shield by 0.85 and deeper shells by 1. For an nd or nf electron, all electrons in groups to its left shield by 1 and others in its own group by 0.35. Outer groups contribute zero. This empirical estimate is orbital-dependent; heavy configurations above Z = 103 use an Aufbau prediction.
Formula
Z effective = Z - S, where S is the sum of Slater shielding contributions from every other electron.
Worked example
For a sodium 3s electron, S = 8 x 0.85 + 2 x 1 = 8.8, so Z effective = 11 - 8.8 = 2.2. A helium 1s electron has S = 0.30 and Z effective = 1.70.
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Frequently asked questions
Why exclude one electron?
An electron does not shield itself. One is subtracted from its own subshell.
Can I choose an empty orbital?
No. Slater rules here estimate shielding of an occupied electron in the neutral configuration.
Is this an exact charge?
No. It is a useful empirical model, not a direct measurement or a full quantum calculation.