Moseley's law

Moseley’s law relates the square root of the frequency of an element’s characteristic X-rays to its atomic number. It established atomic number, rather than atomic mass, as the basis for ordering elements.

Characteristic X-ray: An X-ray emitted when an electron fills an inner-shell vacancy in an atom. These are the spectral lines whose frequencies Moseley measured.
Henry Moseley: An English physicist whose X-ray studies established atomic number as the basis for ordering the elements. His 1913 experiments produced the measurements behind the law.
Nuclear charge: The total positive electric charge of an atomic nucleus, determined by its number of protons. Moseley’s law showed that atomic number tracks the charge governing characteristic X-ray frequencies.
Rydberg formula: A formula that predicts the wavelengths of spectral lines in hydrogen and hydrogen-like atoms. It describes atomic spectral lines, but Moseley’s law compares X-ray frequencies across elements.
Atomic number: The number of protons in an atom’s nucleus, which identifies the element. It is the element-specific quantity related to X-ray frequency.
X-ray spectroscopy of the elements: The measurement of element-specific X-ray emission spectra to identify and compare elements. Moseley applied this method systematically across the periodic table.
Cobalt: A chemical element with atomic number 27, placed before nickel despite having a greater atomic mass. Its position illustrates why atomic number resolves mass-ordering anomalies.
Moseley’s law deviations: Small departures of measured characteristic X-ray frequencies from simple forms of Moseley’s law. These departures reflect effects such as electron screening and relativistic corrections.
X-ray spectroscopy: The study of matter through its interaction with X-rays, including absorption and emission spectra. Moseley’s measurements used characteristic X-ray spectra to test the law.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and grouped by recurring properties. The law supplied a physical basis for the table’s ordering.

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