Henry Moseley finds a link to atomic number

By 1907, when Mendeleev died, chemists were sure that iodine followed tellurium in the Periodic Table and that there was something odd about their relative atomic masses. However no-one was able to measure atomic number, it was just the position of an element in the Periodic Table sequence. For example lithium was known to be the third element but this number three was only because its properties meant that it slotted in between helium and beryllium.

Henry Moseley found and measured a property linked to Periodic Table position. Hence atomic number became more meaningful and the three pairs of elements that seemed to be in the wrong order could be explained.

Moseley used what was then brand-new technology in his experiments. A device now called an electron gun had just been developed. He used this to fire a stream of electrons (like machine gun bullets) at samples of different elements. He found that the elements gave off X-rays. (This is how the X-rays used in hospitals are produced.)

Moseley measured the frequency of the X-rays given off by different elements. Each element gave a different frequency and he found that this frequency was mathematically related to the position of the element in the Periodic Table – he could actually measure atomic number. Click here to see the graphs. You can see that the graph relating frequency to atomic number is a very good straight line while that relating frequency to relative atomic mass is not so good.

The relationship is actually quite complicated. In fact Moseley plotted the square root of the X-ray frequency against atomic number, but do not let this detail obscure how important this result was.

Notice that there are some gaps in the graph where X-ray data are missing – there were more of these in Moseley’s time. Some of them were because it was not easy to fire an electron beam at some elements. Others were because there were still several elements undiscovered when Moseley made his measurements in 1913.