As 2019 is the 150th anniversary of the Mendeleev's Periodic Table, UNESCO has proclaimed this year the International Year of the Periodic Table (IYPT2019). One of the most important steps in the evolution of the Periodic Table is the concept of atomic number, established by Moseley. In this article, his work is commented on, and the emission of X-ray lines is interpreted on the basis of a one-electron transition using Bohr's theory. The agreement is very good for the Ka and Kß lines, which can be used in classes to reinforce the quantized model of atoms. It is also shown that inner-electron-screening effects play a significant role, especially in the L-series lines. In addition to the discussion based on Bohr's theory, the several different X-ray lines that are known for each element are also explained using LS spin--orbit-coupling spectroscopic terms. The presentation of X-ray data and discussion of the data using these two theoretical approaches in both undergraduate and graduate classes resulted in significant grade improvements on the topic of atomic theory.