Moseley & Atomic Number

In 1913, Henry Mosley, the British chemist, found that by bombarding metals with electrons, X-rays are emitted. It was found that electrons were effecting the positive charges in the nucleus and caused different frequencies of X-rays to be emitted. Mosley was able to convert the differences in the frequencies into whole numbers and called them the atomic number. We now know that this number represents the number of protons in the nucleus. This sequencing of atomic numbers allowed the few errors in Mendeleev’s work to be fixed and supported his table as the most accurate and useful chart; allowing it to be adopted as the standard periodic table. Mendeleev’s model had trouble resolving the three main errant sequences of Ar-K, Co-Ni & Te-I because they were arranged by atomic mass.

Though Mendeleev had cobalt and nickel in the right order, further experimentation showed that their atomic masses were incorrectly calculated during the 1800s and they had to be switched. When argon was discovered it appeared to fall between potassium and calcium, though that would throw off the elements around calcium. Mosley’s work straightened out all of the problems. The sequencing of the elements by increasing atomic number is known as atomic law. With this new method of testing, it was also easy to confirm Mendeleev’s hypotheses about missing elements, validating his arrangement and making it the standard for use in labs around the world. Unfortunately, World War I broke out and Mosley’s promising career was cut short when he died in the Battle of Gallipoli in 1915.

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