Q.State Moseley's law, and explain its historical significance for the arrangement of elements in the periodic table.

Moseley measured the $K_\alpha$ characteristic X-ray frequency of many target elements and found $\sqrt{\nu}$ plotted against atomic number $Z$ gives a straight line:

where $a$ depends only on the line concerned and $b$ is a screening constant ($b\approx1$ for $K_\alpha$, since one K-shell electron remains and partly shields the nuclear charge).

Significance. Before this, elements were ordered by atomic MASS, which put a few elements in the wrong chemical family: cobalt (mass $58.93$) had to precede nickel (mass $58.69$); argon ($39.95$) had to precede potassium ($39.10$); tellurium ($127.60$) had to precede iodine ($126.90$). Moseley's measured $Z$ values, ordered strictly by $Z$, placed every one of these correctly -- proving atomic number, not mass, is the periodic table's true ba …

State the law's form, name what $a$ and $b$ represent, then give the historical anomalies it resolve …

• Writing the law as $\nu=a(Z-b)$ instead of $\sqrt{\nu}=a(Z-b)$ (frequency itself is NOT linear in $Z$, its square root is). …