Strontium sits in period 5, group 2 of the periodic table, classifying it as an alkaline earth metal. Its ground-state electron configuration reads [Kr] 5s², or in expanded notation, 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 5s². That outermost shell holds two valence electrons, loosely bound by an effective nuclear charge shielded by 36 core electrons.
Because stripping those two outer electrons requires relatively modest energy, first ionization energy sits at 549.5 kJ/mol, second at 1064.2 kJ/mol, strontium operates exclusively in the +2 oxidation state in its stable compounds. Pure metallic strontium is softer than calcium and cuts easily with a kitchen knife. The moment fresh metal contacts ambient oxygen, it tarnishes into a dull, yellowish oxide film. Exposed to finely divided air, strontium dust ignites spontaneously. When dropped into liquid water, it reacts vigorously, releasing hydrogen gas and forming caustic strontium hydroxide:
Sr (s) + 2 H₂O (l) → Sr(OH)₂ (aq) + H₂ (g)
To prevent continuous decomposition, chemical supply houses store bulk strontium submerged beneath mineral oil or sealed in evacuated argon ampoules.
