How did Henry GJ Mosely's work contribute to the modern periodic table?

Answer 1

HGJ Moseley discovered that there was a basic quantity that correlated better with the properties of elements than atomic weight. He was able to run a series of X-ray spectra, and noticed that there was a regular progression that more or less followed atomic weight, but with a few exceptions. These exceptions turned out to be the same ones where Mendeleev had been having trouble with the periodic law.

Mendeleev had spent a lot of time and energy arguing that tellurium must have a lower atomic weight than iodine, so that tellurium could fit in with selenium and sulfur, and iodine with bromine and chlorine. But the best work available consistently showed that tellurium had an atomic weight about 1% higher than iodine. With Moseley's work it became clear that although tellurium had a higher atomic weight than iodine, it had a lower atomic number. Similarly he had always had tables that assigned the same atomic weight to cobalt and nickel, while the experimental work showed that nickel had an atomic weight about 0.2% lower than cobalt, but an atomic number that was one greater. And the recently discovered noble gas argon had an atomic weight 2% greater than that of the next element, potassium, but an atomic number that was one less. These three exceptions to Mendeleev's periodic law, led to the modern form of the same law: if the elements are arranged in the order of their atomic NUMBER, then their properties repeat in a periodic fashion.