Period

A horizontal row in the periodic table. Elements in the same period have the same number of electron shells (principal quantum number n). Properties change systematically across a period as electrons are added to the same shell with increasing nuclear charge.

Period relationships explain periodic trends: across any period, atomic radius decreases, electronegativity increases, ionisation energy generally increases, and character changes from metallic to metalloid to non-metallic. These trends allow prediction of properties of elements and their compounds.

Within a period, hazard varies from very reactive metals on the left (Na, K — react with water) to reactive nonmetals on the right (Cl, F — toxic, oxidising), with relatively unreactive noble gases at the far right. Period position gives a rough guide to reactivity and hazard type.