The effective nuclear charge is

Z_eff =  Z  -  S

where Z = # protons in the nucleus and S = shielding effect of electrons

Slater's rule is used to determine the value of S  [for electrons in either s- or p- AO only; see link for electrons in d- or f- AO ]

i.  write the electron configuration of the atom in terms of ascending principle quantum number, i.e. electron shell (it does not follow the aufbau principle)

ii.  S = 0.35 (# valence electrons - 1)  +  0.85 (# electrons in next lower electron shell)  +  # remaining electrons

ex. for As, 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³

Z_eff   =  Z  -  S  =  33 - {0.35 (5 - 1)  +  0.85 (18)  +  10} =  6.3

HWproblem:  using Slater's rule, what is the effective nuclear charge of Mg, P, & As  [6 points]