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Alkali metalss, obtaining: physical and chemical properties, use

These metals form simple and stable ionic compounds that, for the most part, are soluble in water; alkali metals often combine with halide, sulfate, carbonate, and silicate ions. However, some elements form a limited number of complex and organometallic compounds.

In general, the physical and chemical properties of alkali metals have been well studied, with the exception of francium due to its radioactive nature.

Physical properties

– The alkali metals are silver, except cesium, which is gold. They conduct electricity and heat, and have low melting points that also decrease with increasing atomic number, because the bond between atoms weakens with increasing atomic radius.

– All elements adopt a cubic structure centered on the body. Because that type of crystal structure is not packed together and the atomic radii of alkali metals are large, they have low densities compared to other metals. In fact, lithium, sodium, and potassium float in water.

– The energy differences between the orbitals of the valence shell of alkali metals coincide with wavelengths of visible light. Due to this, when heating the compounds formed by these metals in a flame, characteristic colors are generated.

– In descending order, the lithium salts give a crimson color, the sodium salts a yellow color, the potassium a violet color, the rubidium a bluish-red color and the cesium a blue color.

All alkali metals have similarities in their chemical properties.

– They have an electronic configuration of valence ns1, which translates into a +1 oxidation state. In turn, this means that they easily lose an electron to reach the noble gas configuration.

– In general, the alkali metals have the lowest ionization energies of all the elements, which also decrease as the atomic radius increases within the group because the valence shell is increasingly distant from the nucleus. This explains the reactivity of alkali metals and the formation of M + ions in aqueous conditions with ease.

– The thermodynamic tendency to form M + ions is confirmed by the standard potentials of the M + / M pairs, which are all large and negative. Such a tendency implies that alkali metals are easily oxidized and can act as powerful reducing agents.

– These elements must be stored in a non-reactive oil to avoid reaction with atmospheric oxygen. Although lithium, sodium, and potassium can be handled in air for short periods, rubidium and cesium must be handled under an inert atmosphere at all times.

Obtaining alkali metals

Sodium

The best way to obtain metallic sodium involves the electrolysis of molten sodium chloride. Because the melting point of sodium chloride is quite high (808 ° C) and a lot of energy is required to hold large amounts of the molten substance, calcium chloride (CaCl2) is often added to lower the melting point to around 600 ° C.

Sodium is obtained in a liquid state at the cathode with a purity of 99.95% under an inert atmosphere.

Lithium and potassium

Lithium can also be obtained from its molten chlorides by electrolysis, but this is not the case for potassium because it is too soluble in chloride (KCl) and does not float on the surface of the cell. Also, it evaporates easily at operating temperature, creating hazardous conditions.

Therefore, potassium is obtained by reducing molten KCl with liquid sodium. The sodium present in the potassium is removed by condensing the vapor at 892 ° C and carrying out a fractional distillation of the liquid metals.

Rubidium and cesium

Electron configuration of cesium. Source: Pumbaa (original work by Greg Robson) / CC BY-SA 2.0 UK (https://creativecommons.org/licenses/by-sa/2.0/uk/deed.en)

Rubidium and cesium can be made in a similar way, but using calcium as the reducing agent. Metallic cesium can also be obtained by electrolysis of molten cesium cyanide (CsCN).

Main uses and applications

Sodium as a reducing agent

As sodium is very easily oxidized, its most important application is as a reducing agent to obtain metals such as titanium, zirconium and hafnium. It is also used as a heat exchanger in nuclear reactors in its liquid state.

Potassium as a fertilizer

Potassium chloride, a fertilizer. Source: Abbas cucaniensis / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Most of the potassium that is processed goes into fertilizers in the form of potassium chloride. In turn, chloride is used in the production of other compounds and solutions such as potassium hydroxide, which is used in the manufacture of liquid soaps.

Potassium cyanide is used for the extraction or coating of metals to facilitate the deposition of copper, silver and gold. Potassium can also be found in some explosives and as a coloring in fireworks.

Lithium as an alloying agent

Metallic lithium is used as an alloying agent in the production of aeronautical and aerospace alloys of aluminum and magnesium. But over time, it has found increasing utility as an anode for alkaline batteries.

Metallic lithium also reduces the melting and sintering temperatures of certain types of glass and ceramics, in addition to reducing their coefficient of expansion. Lithium carbonate is widely used to treat bipolar conditions, while lithium stearate is a widely used lubricant in the automotive industry.

Rubidium and cesium

Rubidium and cesium are often used in the same applications, so one of these elements can be substituted for the other. Applications include glass for fiber optics in the telecommunications industry, night vision equipment, and photoelectric cells. The cesium clock (atomic clock) is used for the international standard time measurement and for the definition of the second.

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