A periodic trend is a systematic variation in an atomic or chemical property across the periodic table. Examples include atomic size, ionization energy, and electronegativity.
Electron-shell filling, screening, and nuclear charge help explain broad patterns, but definitions and exceptions matter. An atomic radius inferred from a covalent bond is not the same measurement as an ionic radius in a crystal, so trends should compare like quantities.
A conventional measure of atomic extent. Covalent, metallic, ionic and van der Waals definitions differ, so one universal hard-sphere radius should not be assumed.
Relative reluctance of a heavier main-group ns electron pair to participate in the higher oxidation state. Retained pairs can nevertheless influence structure through mixing and stereochemical activity.
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