Introduction to Strontium
Strontium (symbol: Sr) is a soft, silvery-white metallic element belonging to Group 2 of the periodic table, known as the alkaline earth metals. It is highly reactive and never found as a free element in nature, always occurring in compounds. Its chemical properties are similar to those of calcium and barium, its neighbors in the periodic table.
Reactivity with Air
Strontium is a highly reactive metal. Upon exposure to air, it readily reacts with both oxygen and nitrogen. This causes the shiny, silvery surface of the metal to quickly tarnish, forming a layer of strontium oxide ($\text{SrO}$) and strontium nitride ($\text{Sr}_3\text{N}_2$). Due to this rapid tarnishing and reactivity, elemental strontium is typically stored under mineral oil or in an inert atmosphere, such as argon, to prevent its reaction with the atmospheric gases.
Reactivity with Water
Strontium reacts vigorously with water, though generally less violently than the alkali metals (Group 1 elements) like sodium or potassium. When strontium metal is placed in water, it reacts to produce strontium hydroxide ($\text{Sr(OH)}_2$) and hydrogen gas ($\text{H}_2$).
The chemical equation for this reaction is: $\text{Sr(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Sr(OH)}_2\text{(aq)} + \text{H}_2\text{(g)}$
The release of hydrogen gas can be observed as bubbles. The reaction generates heat, and if the hydrogen gas concentration is high enough and an ignition source is present, the hydrogen gas can ignite.
Toxicity
Naturally occurring strontium, which comprises several stable isotopes, is generally considered to have low toxicity. Its chemical similarity to calcium means that it can be incorporated into bones and teeth. In fact, small amounts of stable strontium are present in the human body. While high doses of stable strontium compounds ingested over long periods might interfere with calcium metabolism, naturally occurring levels are not typically a concern for human health. It is important to differentiate stable strontium from its radioactive isotope.
Radioactivity
All four naturally occurring isotopes of strontium ($\text{Sr-84, Sr-86, Sr-87, Sr-88}$) are stable and not radioactive. Therefore, naturally occurring strontium is not radioactive.
However, a specific isotope, Strontium-90 ($\text{Sr-90}$), is a highly significant radioactive isotope. It is a fission product from nuclear reactors and nuclear weapons. $\text{Sr-90}$ is a beta emitter with a half-life of approximately 28.8 years. Due to its chemical similarity to calcium, if ingested, $\text{Sr-90}$ can accumulate in bone marrow and bone tissue, where its radioactive decay can damage DNA and lead to serious health issues, including bone cancer and leukemia. This makes $\text{Sr-90}$ a major environmental and health concern in the event of nuclear fallout or contamination.
Flammability
Strontium metal is flammable. When ignited, especially in powdered form or as thin shavings, it burns with a characteristic, brilliant crimson red flame. This distinct color is a result of the emission spectrum of strontium atoms when heated, making it useful in various applications.
Famous Chemical Reaction: Pyrotechnics
One of the most well-known applications of strontium’s chemical properties is its use in pyrotechnics to produce a vivid red color. When strontium compounds, such as strontium nitrate ($\text{Sr(NO}_3)_2$), strontium carbonate ($\text{SrCO}_3$), or strontium chloride ($\text{SrCl}_2$), are heated intensely in a flame (as in fireworks), the strontium atoms absorb energy, causing their electrons to jump to higher energy levels. As these excited electrons return to their original, lower energy levels, they emit light at specific wavelengths, which humans perceive as a brilliant crimson red. This reaction is fundamental to creating the spectacular red displays observed in fireworks used during festivals like Diwali across India.