Henry Moseley

Henry Moseley (1887–1915) was an English physicist whose measurements of characteristic X-ray frequencies showed that atomic number, rather than atomic mass, determines an element’s position in the periodic table.

Moseley's law: An empirical relation between an element’s atomic number and the frequency of its characteristic X-ray lines. It expresses the regular frequency pattern Moseley measured across the elements.
Periodic table: A tabular arrangement of chemical elements that reveals recurring patterns in their properties. Moseley’s ordering by atomic number supplied the table’s modern organizing principle.
Ernest Rutherford: A New Zealand-born physicist who established the nuclear model of the atom and led a major research group. Moseley conducted his X-ray research in Rutherford’s Manchester laboratory.
Cobalt: A metallic chemical element with atomic number 27 and symbol Co. Its position before nickel by atomic number contradicts their slight mass-based ordering.
Characteristic X-ray: An X-ray emitted when an inner-shell vacancy in an atom is filled by an electron from a higher energy level. Moseley identified element-specific characteristic X-ray lines as his measuring signal.
Isotopes: Atoms of the same element with equal proton numbers but different neutron numbers. Their differing masses clarified why atomic mass cannot define an element’s position.
Niels Bohr: A Danish physicist whose early atomic model introduced quantized electron energy levels. Bohr’s theory helped account for the regular X-ray frequencies Moseley observed.
Nickel: A metallic chemical element with atomic number 28 and symbol Ni. Moseley’s ordering placed nickel after cobalt despite nickel’s slightly lower atomic mass.
Atomic number: The number of protons in an atom’s nucleus, defining the identity of a chemical element. Moseley’s measurements established this quantity as the true basis of element order.
Nuclear charge: The positive electric charge of an atomic nucleus, determined by its number of protons. Moseley’s evidence linked element identity to this charge, one unit at a time.

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