Question

How many elements are there in a row between those whose wavelengths of K_α lines are equal to 250 and 179 pm?

Answer 1

We calculate the   values corresponding to the given wavelengths using Moseley’s law.

Substitution gives that 

= 23 corresponding to λ = 250 pm 

and   = 27 corresponding to λ = 179 pm 

There are thus three elements in a row between those whose wavelengths of K_α lines are equal to 250 pm and 179 pm.