# Do low atomic numbers not obey Moseley's law?

## Main Question or Discussion Point

Helium has k-alpha of 24.5 eV whereas if we derive it using Moseley's law, then it is supposed to be 10.2 eV

Also I then looked into many sources and found that Moseley's graph talks about elements having z>=10 only

Later I found in few sources that the assumption that "one electron shields the nucleus and thus we should replace Z by Z-1 in Rydberg formula" is not entirely correct
$$E=13.6(Z-1)^2(\frac{1}{1^2}-\frac{1}{2^2})$$
as the effect due to shielding is little less than expected and should be taken into account.

My question is why is this effect (that K-shell electron's shielding effect is less than 1) not applicable as we move up in the periodic table i.e. for z>=10

## Answers and Replies

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Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

Ok i will try to reword my question. Why does Moseley's law applicable only on z>10. What property changes after z=10.