Moseley derived his formula empirically by fitting the square root of the X-ray frequency plotted against the atomic number. This formula can be explained based on the Bohr model of the atom, namely, E = h ν ν = E i − − E f = m e q e 2 q Z 2 8 h 2 ε ε 0 2 ( 1 n f 2 − − 1 n i 2 ) , where