[YuliaSnow]

I am trying to calculate the effective nuclear charge on a outermost electron of a potassium ion (K⁺).

How many core electrons are in a potassium ion?
I just need to check this, I think it is 10.

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My solution

Z_eff=Z-s
Where Z=protons
and s=core electrons

I believe the number of core electrons is 10 because in a neutral atom of potassium there is 19 electrons and 18 core electrons. Shell 4 where 1 electron originally was becomes 0 when it is a potassium ion. Therefore by removing electron one, the core electrons are the 10 in shell 1 and 2 and 8 is in our outermost shell (not valence).

Z_eff=19-10=9+

The effective nuclear charge on the outermost electron in a potassium ion will be 9+.
Logically this makes sense because in Argon, the Z_eff=8+ and because potassium has one additional proton, it should be higher, electron configuration is the same in the potassium ion and argon.
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Did I do this right?

Thanks in advance

[AdiDex]

Nope you are wrong ,
By your logic Na₁₁ also have effective nuclear charge equals to 10 , why only Na , all elements from Na  to Cl will have same effective nuclear charge....

[YuliaSnow]

Nope you are wrong ,
By your logic Na₁₁ also have effective nuclear charge equals to 10 , why only Na , all elements from Na  to Cl will have same effective nuclear charge....

Can you explain what you mean?

I know this is not the case.

Na - 11-10=1
Si - 14-10=4
P - 15-10=5
S - 16-10=6
Cl - 17-10=7

I am saying talking about a potassium ion (K⁺). It has one less electron, in the question it asks for outermost electron. In this case the outermost will be in the n=3 shell, so the core electrons will be in n=1,2 (2+8=10)

So K⁺ - 19-10=9