Moseleon

Meet Moseleon, an atom who shoots out an X-ray when an electron drops into an inner shell.

Knock an electron out of the innermost shell of an atom, and an electron from the next shell drops in to fill the gap, giving out an X-ray. In 1913 Henry Moseley measured these X-rays for many elements. He found that the square root of their frequency rises in a straight line with the atomic number Z, √f ∝ Z − 1.

The 1 is there because the remaining inner electron partly shields the nucleus. For copper, with Z = 29, the main X-ray line has an energy of about 8.0 keV, close to the simple estimate of ¾ × 13.6 eV × 28².

Moseley's result showed that atomic number is a real physical quantity, the charge of the nucleus, and not just a place in a list.

It let chemists sort the periodic table by atomic number and spot gaps for elements that had not yet been found.