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Using Slater's rule I calculated the effective nuclear charge on 3p electron of aluminium as:

Config: $\ce{1s^2 (2s^2 2p^6) (3s^2 3p^1)}$

$S = 2(0.35) + 8(0.85) + 2 = 9.5$

$Z_{eff} = Z - S = 3.5$

The Wikipedia page states its value as 4.066.

Which one is correct? Did I go wrong somewhere?

Joy
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    You're doing it correctly. The value you cite is from E. Clementi and D. L. Raimondi. "Atomic Screening Constants from SCF Functions." Journal of Chemical Physics, volume 38, number 11, 1963, pp. 2686–2689. They used a very primitive (by today's standards) method for computing the screening constants. It is entirely possible that their number is off. It's also known that Slater's rule is good for the first 2 rows, then starts to diverge from experiment from there, so aluminum falls into that category. – Todd Minehardt Aug 13 '21 at 22:06

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