The compound with the chemical formula \(\text{SrI}_2\) is named Strontium Iodide. This substance is an inorganic salt and a binary halide, composed of the alkaline earth metal strontium (\(\text{Sr}\)) and the halogen iodine (\(\text{I}\)). It is formed from the ionic combination of these two elements. The compound is typically encountered as a white, crystalline solid known for its high solubility in water.
Nomenclature and Ionic Formation
The name Strontium Iodide is derived from the chemical nomenclature rules for binary ionic compounds. Strontium (\(\text{Sr}\)), an alkaline earth metal found in Group 2 of the periodic table, is named first and unchanged. Iodine (\(\text{I}\)), a halogen, is modified with the suffix “-ide,” resulting in “iodide.”
Strontium readily loses its two valence electrons, forming the cation \(\text{Sr}^{2+}\). Iodine tends to gain a single electron, forming the anion \(\text{I}^{-}\).
For the compound to be electrically neutral, the total positive charge must balance the total negative charge. Since the strontium cation has a \(2+\) charge and the iodide anion has a \(1-\) charge, two iodide ions (\(\text{I}^{-}\)) are required to neutralize the single \(\text{Sr}^{2+}\) ion. This necessary \(1:2\) ratio dictates the chemical formula \(\text{SrI}_2\). Because strontium has a fixed \(2+\) charge, Roman numerals are not used in its name.
Distinct Physical and Chemical Properties
Strontium Iodide is a colorless to white crystalline solid in its anhydrous form, though it can also exist as a hexahydrate, \(\text{SrI}_2 \cdot 6\text{H}_2\text{O}\). The anhydrous form has a relatively low melting point for an ionic salt, typically reported around \(515 \text{ }^\circ\text{C}\). The crystal structure of anhydrous Strontium Iodide is typically orthorhombic or hexagonal, depending on the temperature.
The compound is highly soluble in water, dissolving at a rate of approximately \(177\) grams per \(100 \text{ mL}\) of water at \(20 \text{ }^\circ\text{C}\). Furthermore, the compound is highly hygroscopic and deliquescent, meaning it readily absorbs moisture from the surrounding air. This absorption can cause the solid crystals to dissolve.
When exposed to air, Strontium Iodide can slowly develop a yellowish color. This discoloration results from a slight decomposition that releases free iodine onto the crystal surface. At high temperatures, especially in the presence of air, the compound decomposes completely into strontium oxide and free iodine.
Key Technological Applications
The most prominent modern application for Strontium Iodide is in the field of radiation detection, where it is used as a high-performance scintillator material. A scintillator is a substance that emits light when struck by high-energy radiation, such as gamma rays or X-rays.
Scintillation Properties
Strontium Iodide is primarily used when doped with trace amounts of europium, creating \(\text{SrI}_2(\text{Eu})\). The addition of europium enables high-efficiency light emission, transforming absorbed radiation energy into measurable photons.
The material is highly valued in gamma-ray spectroscopy because it produces a very high light yield, often exceeding \(80,000\) photons per mega-electron volt (\(\text{MeV}\)) of absorbed energy. This exceptional light output results in excellent energy resolution, allowing detectors to distinguish between gamma rays of similar energy with high precision. This capability makes it an ideal choice for use in handheld radioisotope identification devices (RIDs) and other security or medical instruments requiring accurate isotopic analysis.
Other Uses
Strontium Iodide has also seen minor and historical uses. It was occasionally used in medicine as a substitute for potassium iodide. Historically, it was also used in certain photographic processes to increase image contrast.