How did Mosley's work on atomic number show that atomic number is a fundamental property better than atomic weight?

Explanation

Mosley (1913) obtained the X-ray spectrum of different elements and found that the frequency `nu` of the lines in the spectrum is related with the atomic number (Z) of the element, as `sqrt(nu) = a(Z-b)`
a,b are constants for any selected series of lines in the X-ray spectrum (`K,L,M`...........etc. series) of the element. The element is used as anticathode in the X- ray tube.
Z - at. no. of the element (used as anticathode).
When Mosley plotted a graph between `sqrt(nu) xx 10^(8)` (on Y-axis) and the atomic number Z (on X-axis) of the elements fro which the X-rays were obtained, he got a straight line.
He also plotted a graph between `sqrt(nu) xx 10^(8)` and atomic weight of the element. He did not get a straight line.

These observations indicated that atomic number is a fundamental property of an atom and not its atomic weight. Actually the frequency `(nu)` of X-rays depends on the internal structure of the atom which is related to the number of electron in the atom (atomic number).