Henry Moseley and the discovery of atomic number

The trouble with atomic weight

For more than 40 years after Mendeleev, the periodic table was ordered by atomic weight, with a few awkward exceptions. Argon is heavier than potassium, cobalt is heavier than nickel and tellurium is heavier than iodine, yet in each pair the heavier element clearly belongs first. Chemists also could not tell how many rare earth elements were still waiting to be found, because their atomic weights were so close together.

In 1911 Ernest Rutherford showed that an atom has a tiny, positively charged nucleus. Soon afterward the Dutch scientist Antonius van den Broek suggested that an element’s position in the table might equal the charge on that nucleus. Moseley set out to test the idea.

Moseley’s X-ray experiment

Henry Gwyn Jeffreys Moseley, born in 1887, joined Rutherford’s laboratory at the University of Manchester after studying at Oxford. He used samples of different elements as targets in an X-ray tube and measured the wavelengths of the X-rays each one gave off by reflecting them from a crystal, a technique recently developed by William Henry Bragg and his son Lawrence. In late 1913 he published results for the elements from calcium to zinc, and in 1914 he extended the work to most of the elements from aluminum to gold.

The result was remarkably simple. The square root of the frequency of each element’s strongest X-ray line rose by almost the same step from one element to the next. This relationship is now known as Moseley’s law.

Atomic number takes over

Moseley concluded that each element has one more unit of positive charge in its nucleus than the element before it. That whole number, the atomic number, is what really fixes an element’s place; today we know it equals the number of protons in the nucleus. Ordering by atomic number put argon before potassium, cobalt before nickel and tellurium before iodine with no exceptions needed, and the periodic law was restated: properties repeat with atomic number, not atomic weight.

Gaps revealed

Because atomic numbers are whole numbers, Moseley’s method could count the elements. It showed that elements with atomic numbers 43, 61, 72 and 75 were still missing, and it made it possible to settle how many lanthanides exist between lanthanum (57) and lutetium (71). The gaps were filled over the next three decades: hafnium (72) in 1923, rhenium (75) in 1925, technetium (43) in 1937 and promethium (61) in 1945. Hafnium and rhenium were both tracked down using X-ray spectroscopy, the method Moseley had pioneered.

Death at Gallipoli

When World War I began in 1914, Moseley volunteered for the British Army and became a signals officer in the Royal Engineers. He was killed in action at Gallipoli, in present-day Turkey, on August 10, 1915, at the age of 27. Rutherford wrote his obituary in the journal Nature, and many scientists regarded his death as one of the war’s greatest losses to science. In only a few years of research, Moseley had given the periodic table the foundation it still rests on.