the energy of characteristic x-rays is the energy difference between two electron in different shells. it is well defined and dependent on the atomic number of the atom. for example, the energy of the x-ray K.. line is the energy difference between a K shell electron and L shell electron. thus, we can identify a chemical element from the characteristic x-ray that it emits. moseley's Law defines the relationship between wavelength of characteristic x-rays ..lamda.. and atomic number(Z).