A piece of strontium burns in contact with air as a consequence of its high reactivity, and since it has an electronic configuration of the ns2 type, it easily gives up its two valence electrons, especially to the diatomic oxygen molecule.
If the surface area of the metal is increased by pulverizing it into a finely divided powder, the reaction occurs immediately, and even burns with an intense reddish flame. Strontium, the metal that participates in this reaction, is a metal in group 2 of the periodic table.
This group is made up of the elements known as alkaline earths. The first of the elements that leads the group is beryllium, followed by magnesium, calcium, strontium, barium, and finally, radium. These elements are metallic in nature and, as a mnemonic to remember them, the expression can be used: “Mr. Becambara ”.
The “Sr” to which the expression refers is none other than strontium metal (Sr), a very reactive chemical element that is not naturally found in its pure form, but rather combined with other elements of the environment or its environment to give rise to its salts, nitrides and oxides.
For this reason, minerals and strontium oxide are the compounds in which strontium is found in nature.
Physical and chemical properties of strontium oxide
Strontium oxide is a white, porous and odorless solid compound and, depending on its physical treatment, it can be found on the market as a fine powder, as crystals or as nanoparticles.
Its molecular weight is 103.619 g / mol and it has a high refractive index. It has high melting points (2531ºC) and boiling points (3200ºC), which results in strong bonding interactions between strontium and oxygen. This high melting point makes it a thermally stable material.
It is a highly basic oxide; This means that it reacts at room temperature with water to form strontium hydroxide (Sr (OH) 2):
SrO (s) + H2O (l) → Sr (OH) 2
It also reacts or retains moisture, an essential characteristic of hygroscopic compounds. Therefore, strontium oxide has a high reactivity with water.
In other solvents — for example, alcohols like drugstore ethanol or methanol — it is slightly soluble; while in solvents such as acetone, ether or dichloromethane, it is insoluble.
Why is it like this? Because metal oxides – and even more those formed from alkaline earth metals – are polar compounds and therefore interact to a better degree with polar solvents.
It can not only react with water, but also with carbon dioxide, producing strontium carbonate:
SrO (s) + CO2 (g) → SrCO3 (s)
Reacts with acids – such as dilute phosphoric acid – to produce the phosphate salt of strontium and water:
3SrO (s) + 2 H3PO4 (dil) → Sr3 (PO4) 2 (s) + 3H2O (g)
These reactions are exothermic, which is why the water produced evaporates due to the high temperatures.
The chemical structure of a compound explains how its atoms are arranged in space. In the case of strontium oxide, it has a gem-salt crystalline structure, the same as table salt or sodium chloride (NaCl).
Unlike NaCl, a monovalent salt —that is, with cations and anions of one magnitude of charge (+1 for Na and -1 for Cl) -, SrO is divalent, with charges of 2+ for Sr, and -2 for O (O2-, oxide anion).
In this structure, each O2- ion (red) is surrounded by six other bulky oxide ions, accommodating the smaller Sr2 + ions (green) in its resulting octahedral interstices. This packing or arrangement is known as a face-centered cubic unit cell (ccc).
The chemical formula of strontium oxide is SrO, but it does not absolutely explain the chemical structure or the type of bond that exists.
In the previous section it was mentioned that it has a rock-salt-like structure; that is, a very common crystalline structure for many salts.
Therefore, the type of bond is predominantly ionic, which would clarify why this oxide has high melting and boiling points.
As the bond is ionic, electrostatic interactions hold the strontium and oxygen atoms together: Sr2 + O2-.
If this bond were covalent, the compound could be represented by bonds in its Lewis structure (omitting the unshared oxygen electron pairs).
The physical properties of a compound are essential to predict what would be its potential applications in industry; therefore, these are a macro reflection of its chemical properties.
Strontium oxide, thanks to its high thermal stability, finds many applications in the ceramic, glass and optical industries.
Its use in these industries is mainly intended to replace lead and be an additive that confers better colors and viscosities to the raw material of the products.
What products? The list would have no end, because in any of these that has glasses, enamels, ceramics or crystals in any of its pieces, strontium oxide may be useful.
As it is a very porous solid, it can intercalate smaller particles, and thus provide a range of possibilities in the formulation of materials, so light as to be considered by the aerospace industry.
That same porosity allows it to have potential uses as a catalyst (accelerator of chemical reactions) and as a heat exchanger.
Strontium oxide also serves as a source for the production of pure strontium for electronic purposes, thanks to the metal’s ability to absorb X-rays; and for the industrial preparation of its hydroxide, Sr (OH) 2, and its peroxide, SrO2.
It is a corrosive compound, so it can cause burns with simple physical contact in any part of the body. It is very sensitive to humidity and must be stored in dry and cold spaces.
The salts product of the reaction of this oxide with different acids behave in the organism like calcium salts, and are stored or expelled by similar mechanisms.
Currently, strontium oxide by itself does not pose major health risks.