Effective nuclear charge is the charge felt by valence electrons of multi-electron atom taking into consideration the number of shielding electrons that surround the nucleus. To calculate Z eff for electron we use this formula : Z eff = Z - S where Z is the number of protons in nucleus and S is the average amount of electron density between the nucleus and electron in consideration so Z eff = 17 - 10 (core electrons) = 7 So the correct answer is C
