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Ionization energy of sodium = 5.14 eV = 8.23e-19 J

Zeff = Z - S

I tried using the equation Zeff = Z - S, but when I followed all the rules, I found

**S = (8*0.85)+(2*1.00) = 8.8**

**Zeff = 11 - 8.8 = 2.2**

but the answer in my book says that it should be 1.8, and that method didn't use the ionization energy of sodium.

__I need to find an equation that explains quantitatively how ionization energy relates to Zeff.__