Strontium bromide is a
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
with a formula SrBr
2. At room temperature it is a white, odourless, crystalline powder. Strontium bromide imparts a bright red colour in a
flame test
A flame test is an analytical procedure used in chemistry to detect the presence of certain elements, primarily metal ions, based on each element's characteristic flame emission spectrum (which may be affected by the presence of chloride io ...
, showing the presence of strontium ions. It is used in
flares and also has some pharmaceutical uses.
Preparation
SrBr
2 can be prepared from
strontium hydroxide
Strontium hydroxide, Sr(OH)2, is a caustic alkali composed of one strontium ion and two hydroxide ions. It is synthesized by combining a strontium salt with a strong base. Sr(OH)2 exists in anhydrous, monohydrate, or octahydrate form.
Prepar ...
and
hydrobromic acid
Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide (HBr) in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at and contains 47.6% HBr by mass, which is 8.77 mol/L. H ...
.
:
Alternatively
strontium carbonate
Strontium carbonate (SrCO3) is the carbonate salt of strontium that has the appearance of a white or grey powder. It occurs in nature as the mineral strontianite.
Chemical properties
Strontium carbonate is a white, odorless, tasteless powder. B ...
can also be used as
strontium
Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
source.
:
These reactions give
hexahydrate
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was underst ...
of SrBr
2, which decomposes to dihydrate at 89 °C. At 180 °C
anhydrous
A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...
SrBr
2 is obtained.
[Dale L. Perry, Sidney L. Phillips: ''Handbook of Inorganic Compounds''. CRC Press, 1995, , ().]
Structure
At room temperature, strontium bromide adopts a
crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...
with a
tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square ...
unit cell
In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...
and
space group
In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...
''P''4/''n''. This structure is referred to as α-SrBr
2 and is
isostructural Isostructural chemical compounds have similar chemical structures. "Isomorphous" when used in the relation to crystal structures is not synonymous: in addition to the same atomic connectivity that characterises isostructural compounds, isomorphous s ...
with
EuBr2 and
USe2. Around 920 K (650 °C), α-SrBr
2 undergoes a
first-order
In mathematics and other formal sciences, first-order or first order most often means either:
* "linear" (a polynomial of degree at most one), as in first-order approximation and other calculus uses, where it is contrasted with "polynomials of high ...
solid-solid
phase transition
In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of ...
to a much less ordered phase, β-SrBr
2, which adopts the cubic
fluorite
Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.
The Mohs sca ...
structure. The beta phase of strontium bromide has a much higher
ionic conductivity of about 1 S cm
−1, comparable to that of molten SrBr
2, due to extensive disorder in the bromide sublattice.
Strontium bromide melts at 930 K (657 °C).
File:Strontium-bromide-xtal-3D-sf.png, Space-filling model
In chemistry, a space-filling model, also known as a ''calotte model'', is a type of three-dimensional (3D) molecular model where the atoms are represented by spheres whose radii are proportional to the radii of the atoms and whose center-to- ...
of the packing of Sr2+ and Br− ions in α-SrBr2
File:Strontium-bromide-xtal-Sr1-coordination-view-2-3D-bs-17-25.png, Distorted square antiprismatic coordination geometry
The term coordination geometry is used in a number of related fields of chemistry and solid state chemistry/physics.
Molecules
The coordination geometry of an atom is the geometrical pattern formed by atoms around the central atom.
Inorganic coo ...
of crystallographically independent strontium atom number 1
File:Strontium-bromide-xtal-Sr2-coordination-view-2-3D-bs-17-25.png, Square antiprismatic coordination geometry of strontium number 2
File:Strontium-bromide-xtal-Br1-coordination-3D-bs-17-25.png, Flattened tetrahedral coordination geometry of bromine number 1
File:Strontium-bromide-xtal-Br2-coordination-3D-bs-17-25.png, Distorted tetrahedral coordination geometry of bromine number 2
File:Strontium-bromide-xtal-Br3-coordination-3D-bs-17-25.png, Tetrahedral coordination geometry
In a tetrahedral molecular geometry, a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1(−) = 109.4712206...° ≈ 109.5° when all four substituents are ...
of bromine number 3
File:Strontium-bromide-xtal-Br4-coordination-3D-bs-17-25.png, Tetrahedral coordination geometry of bromine number 4
See also
*
Strontium chloride
Strontium chloride (SrCl2) is a salt of strontium and chlorine.
It is a 'typical' salt, forming neutral aqueous solutions. As with all compounds of strontium, this salt emits a bright red colour in flame, and is commonly used in fireworks to that ...
References
* http://www.webelements.com/
Strontium compounds
Bromides
Alkaline earth metal halides
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