Atomic number explained from atomic structure

Within ten years of Moseley’s work, the structure of the atom was further unravelled and atomic number seen to be the number of protons in the nucleus of an atom. Some people call Z the proton number, but it could have been called the Moseley number.

The X-rays given out by atoms bombarded with electrons are formed as follows.

Imagine an atom of magnesium. It has the electron arrangement 2, 8, 2 as shown in the animation below [click to start].

If one of the electrons from the electron gun hits one of the electrons in the inner shell it may knock the inner electron out of the atom completely leaving a gap in the inner shell [click here].

Now, one of the electrons in shell 2 may be pulled by the attraction of the nucleus into shell 1 to fill the gap, [click here].

As it does so, it gives out energy in the form of X-rays. 

Production of X-rays - click to animate: <previous | next>

If the same process happened for another atom with more protons in its nucleus, more energy would be given out because the electron would be attracted more strongly to the nucleus. In general, the bigger the charge on the nucleus (ie the bigger the atomic number), the more energy the X-rays will have. Like all electromagnetic radiation, the energy of an X-ray is related to its frequency.

The X-rays Moseley used were caused by electrons falling from shell 2 into shell 1. The reason that the energy (and therefore wavelength) is linked to the atomic number (the positive charge on the nucleus) is that the greater the charge on the nucleus, the more the electrons are attracted to it and the more energy they give out when dropping from one shell to another.