Question

Total number of elements which are present in a row on the periodic table between those elements, whose wavelength of $K_{\alpha }$ lines are equal to $250$ and $179 \, pm$ are (Rydberg constant = $1.097 \, \times \, 10^{7} \, m^{- 1}$ )

Answer 1

Wavelength of $K_{\alpha }$ line is given by
$\lambda =\frac{4}{3 R \left(Z - 1\right)^{2}}$
For $\lambda $ , between $250 \, $ and $179 \, pm$ three elements are possible
( $Z \, = \, 24$ , $25$ and $26$ ).