Atomic number
The number of protons in an atom’s nucleus. It identifies the chemical element, regardless of how many neutrons or electrons the atom has.
Proton: A positively charged subatomic particle found in atomic nuclei. Each proton in the nucleus contributes one unit to the atomic number.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and recurring properties. Its ordering makes atomic number the organizing principle of the elements.
Henry Moseley: An English physicist whose X-ray measurements established atomic number as a measurable property of elements. His work showed that atomic number, rather than atomic weight, determines elemental order.
Mendeleev’s predicted elements: Elements Mendeleev anticipated from gaps in his periodic table, including gallium, scandium, and germanium. Their later discovery supported periodic classification, whose ordering atomic number clarified.
Atomic mass: The mass of an atom, usually expressed in unified atomic mass units. Atomic mass depends on nuclear constituents and isotopic composition, not proton count alone.
Neutron: An electrically neutral subatomic particle found in most atomic nuclei. Neutrons change an atom’s mass number without changing its atomic number.
Chemical element: A pure substance whose atoms all have the same number of protons. Atomic number defines which element an atom belongs to.
Antonius van den Broek: A Dutch amateur physicist who proposed that an element’s place in the periodic table corresponds to nuclear charge. His proposal anticipated the nuclear interpretation later confirmed by Moseley.
Tellurium: A chemical element with atomic number 52, placed before iodine despite having a greater atomic mass. Its position illustrates why atomic number gives a better elemental order than atomic weight.
Relative atomic mass: The weighted mean mass of an element’s atoms relative to one twelfth the mass of a carbon-12 atom. Unlike atomic number, it can be nonintegral because it averages naturally occurring isotopes.