Henry Mosely, in 1914, found a way to measure a property that linked perfectly to periodic table placement. Using an electro gun, he was able to fire a line of electrons at different element samples. By doing so, he found that different elements give off a certain X-ray. He then measured the frequency of the element and realised that the frequency of the X-ray was linked to the positioning of the element on the Periodic Table. The atomic number was actually measurable!