• question_answer A cobalt (atomic no. = 27) target is bombarded with electrons, and the wavelengths of its characteristic x-rays spectrum are measured. A second weak characteristic spectrum is also found, due to an impurity in the target. The wavelength of the ${{K}_{a}}$ lines are 225.0 pm (cobalt) and 100.0 pm (impurity). Atomic number of the impurity is (take b=1) A) 39 B) 40    C) 59 D) 60

Using Mosely?s law for both cobalt and impurity $\sqrt{f}=K\,\,(Z-1)\Rightarrow \sqrt{\frac{c}{\lambda }}=K\,\,(Z-1)$$\Rightarrow \sqrt{\frac{c}{{{\lambda }_{co}}}}=K({{Z}_{co}}-1)$and $\sqrt{\frac{c}{{{\lambda }_{x}}}}=K({{Z}_{x}}-1)$$\Rightarrow \sqrt{\frac{{{\lambda }_{co}}}{c}}=\frac{{{Z}_{x}}-1}{{{Z}_{co}}-1}={{Z}_{x}}=40$