X-rays emitted from a copper target and a molybdenum target are found to contain a line of wavelength 22.85 nm attributed to the Kα line of an impurity element. The Kα lines of copper (Z=29) and molybdenum (Z=42) have wavelength 15.42 nm and 7.12 nm respectively. Using Moseley’s law, γ^1/2 = a(Z-b) calculate the atomic number of the impurity element.

Dear student,

K-alpha emission lines result when an electron transitions to the innermost "K" shell (principal quantum number 1) from a 2p orbital of the second or "L" shell (with principal quantum number 2). The line is actually a doublet, with slightly different energies depending on spin-orbit interaction energy between the electron spin and the orbital momentum of the 2p orbital. K-alpha is typically by far the strongest X-ray spectral line for an element bombarded with energy sufficient to cause maximally intense X-ray emission.