Understanding Strontium
Strontium (Sr) is a chemical element with atomic number 38, located in Group 2 of the periodic table, known as the alkaline earth metals. It is a soft, silvery-white metal that is highly reactive. Strontium is never found in its elemental form in nature, always occurring as part of mineral compounds, such as strontianite (strontium carbonate) and celestine (strontium sulfate).
Reactivity of Strontium
Reactivity with Water
Strontium reacts vigorously with water. When elemental strontium comes into contact with water, it rapidly displaces hydrogen, forming strontium hydroxide and hydrogen gas. The reaction is exothermic, meaning it releases heat:
Sr(s) + 2H₂O(l) → Sr(OH)₂(aq) + H₂(g)
This reactivity is greater than that of magnesium (above it in Group 2) but less than that of barium (below it). The hydrogen gas produced can ignite, especially if the reaction is carried out with powdered strontium or in a confined space. This vigorous reaction makes elemental strontium unsuitable for direct handling without proper safety precautions.
Reactivity with Air
Strontium is highly reactive with air. Upon exposure to air, its shiny, silvery surface rapidly tarnishes, forming a dull, yellowish-white layer of strontium oxide (SrO) and strontium nitride (Sr₃N₂). The reaction with oxygen in the air is:
2Sr(s) + O₂(g) → 2SrO(s)
Due to this high reactivity, elemental strontium must be stored under mineral oil or in an inert atmosphere, such as argon, to prevent its reaction with atmospheric oxygen and moisture.
Safety Considerations
Toxicity
Naturally occurring, stable strontium isotopes are generally considered non-toxic in their compound forms at typical environmental levels. The human body can absorb stable strontium, and it behaves chemically similar to calcium. Consequently, a small amount of strontium is normally present in bones and teeth. However, high levels of stable strontium ingestion over long periods can potentially interfere with bone development, especially in children, although such occurrences are rare.
Radioactivity
While stable strontium is not radioactive, the isotope Strontium-90 ($^{90}$Sr) is highly radioactive and of significant concern. Strontium-90 is a byproduct of nuclear fission, generated in nuclear reactors and nuclear weapons testing. It has a relatively long half-life of approximately 28.8 years. Due to its chemical similarity to calcium, Strontium-90 can be absorbed by the body and deposited in bones, where it emits high-energy beta radiation. This internal radiation exposure can damage bone marrow and surrounding tissues, increasing the risk of bone cancers and leukemia. Following nuclear incidents like Chernobyl in Ukraine or Fukushima in Japan, Strontium-90 was a significant component of radioactive fallout, leading to long-term environmental contamination and health monitoring efforts in affected regions.
Flammability
Elemental strontium metal is flammable. It can ignite spontaneously in powdered form upon exposure to air or if heated. Once ignited, it burns with a distinctive bright red flame. For this reason, special fire extinguishing agents, typically Class D dry chemical extinguishers designed for metal fires, are required if strontium ignites. Water or common fire extinguishers containing water or carbon dioxide should not be used as they can exacerbate the fire.
Illustrative Chemical Reaction
Pyrotechnics
One of the most well-known applications of strontium compounds involves their use in pyrotechnics to produce vibrant colors in fireworks. When strontium salts, such as strontium nitrate (Sr(NO₃)₂) or strontium carbonate (SrCO₃), are heated to high temperatures in a flame, the electrons in the strontium atoms become excited. As these electrons return to their ground state, they emit light at specific wavelengths. For strontium, this emitted light is predominantly in the red region of the electromagnetic spectrum, producing a brilliant crimson red color. This chemical reaction is responsible for the striking red displays seen in fireworks celebrations globally, from Diwali festivities in India to New Year’s Eve spectaculars in major cities worldwide.