Henry Moseley and the Atomic Numbers

Henry Moseley (1887-1915)

For sure you do remember that poster from your classroom with all the chemical elements ordered in the so-called periodic table. But, certainly only a few of you will have heard about Henry Moseley and his concept of the atomic numbers. In chemistry and physics, the existence of a periodic table creates an ordering for the elements, and was first proposed by Russian chemist and inventor Dimitri Mendeleev. The atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. The atomic number uniquely identifies a chemical element. This discovery is thanked to the British physicist Henry Moseley, who justified this empirical and chemical concept of the atomic number also from physical laws. Henry Moseley discovered that the wavelength (energy) of an X-ray depended upon the nuclear charge of an atom.

In 1913, almost fifty years after Mendeleev has proposed his periodic table of chemical elements, Henry Moseley at the University of Manchester observed and measured the X-ray spectra of 10 elements that occupied consecutive places in the periodic table. These spectra were found by the method of diffraction of X-rays through crystals. Moseley discovered a systematic mathematical relationship between the wavelengths of the X-rays produced in the diffraction process by the targeted elements and their atomic numbers. He concluded that there was

a fundamental quantity which increases by regular steps as we pass from one element to the next.

This relationship has become known as Moseley’s law. In 1920, Ernest Rutherford at Cambridge identified this quantity as the atomic number.

Moseley’s formula, on account of Nobel laureate Niels Bohr, not only established atomic number as a measurable experimental quantity, but gave it a physical meaning as the positive charge on the atomic nucleus, i.e. the number of protons of the nucleus. With the discovery of isotopes of the elements, it became apparent that atomic weight was not the significant player in the periodic law as Mendeleev and others had proposed, but rather, the properties of the elements varied periodically with atomic number. When atoms were arranged according to increasing atomic number, the few problems with Mendeleev’s periodic table had disappeared. Because of Moseley’s x-ray work, elements could be ordered in the periodic system in order of atomic number rather than atomic weight.

At yovisto you might learn more about Henry Moseley and his concept of the atomic numbers in a lecture from Prof. Donald Sadoway of MIT on ‘Introduction to Solid State Chemistry‘.

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