Strontium (Sr)

Strontium has the ground-state configuration \([Kr]5s^2\). Like other alkaline-earth metals, it most commonly forms the +2 oxidation state as \(\mathrm{Sr^{2+}}\), by losing its two 5s electrons.

With water (vigorous, more reactive than Ca):

\(\mathrm{Sr(s) + 2\,H_2O(l) \rightarrow Sr(OH)_2(aq) + H_2(g)}\)

In air, freshly cut Sr tarnishes rapidly, forming oxides/peroxides:

\(\mathrm{2\,Sr + O_2 \rightarrow 2\,SrO}\)

It’s stored under mineral oil or inert gas to prevent oxidation.

Heating excites Sr atoms/ions; when electrons relax, they emit characteristic red lines in the visible spectrum. In pyrotechnics, red color mainly comes from Sr²⁺ salts (e.g., SrCO₃, Sr(NO₃)₂) that produce intense emission lines perceived as crimson.

• SrCO₃ (strontianite): sparingly soluble; used in ceramics/pyrotechnics.
• SrSO₄ (celestine): very low solubility; main natural mineral source.
• Sr(OH)₂: a strong base (more soluble than Ca(OH)₂).
• SrO: basic oxide; reacts with CO₂ to give carbonate.

Calcination: heating carbonate to oxide:

\(\mathrm{SrCO_3(s) \xrightarrow{\Delta} SrO(s) + CO_2(g)}\)

Carbonation: oxide absorbing carbon dioxide:

\(\mathrm{SrO(s) + CO_2(g) \rightarrow SrCO_3(s)}\)

• Pyrotechnics: Sr salts (SrCO₃, Sr(NO₃)₂) for red flames.
• Magnets: Strontium ferrite \(\mathrm{SrFe_{12}O_{19}}\) in hard ceramic magnets (speakers, motors).
• Optics/electronics: SrTiO₃ (perovskite) in capacitors and as a substrate for oxide electronics; phosphors such as SrAl₂O₄:Eu for long-afterglow pigments.

Stable isotopes include ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr. ⁹⁰Sr is a beta-emitting fission product with biological concern because Sr behaves like Ca in the body and can be incorporated into bone. Geochemists use ⁸⁷Sr/⁸⁶Sr ratios (with radiogenic ⁸⁷Sr from ⁸⁷Rb decay) for rock and water provenance studies.

Yes. Sr²⁺ is chemically similar to Ca²⁺ and can substitute at limited levels in bone mineral. This similarity explains both some therapeutic explorations and the radiological hazard of ⁹⁰Sr. Dietary/medical use must be carefully controlled and evaluated by professionals.

For sulfates, solubility decreases down the group: MgSO₄ > CaSO₄ > SrSO₄ > BaSO₄. Thus SrSO₄ is quite insoluble. For hydroxides, solubility increases down the group; Sr(OH)₂ is more soluble/stronger as a base than Ca(OH)₂.

Metallic Sr is highly reactive—keep under dry mineral oil or inert gas, avoid moisture and acids, and use non-sparking tools. In case of fire, use a Class D extinguishing agent; do not use water or CO₂. Compounds of natural (stable) Sr are generally low-toxicity, but dust should still be minimized.