Potassium Chloride Crystal Structure

Written by april kohl
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Potassium Chloride Crystal Structure
In a face-centred cubic lattice, potassium atoms are surrounded by chlorine atoms, and vice versa. (Strontium molecule image by Vladislav Gajic from Fotolia.com)

Potassium chloride is a metal halide salt commonly derived from potash and used for a variety of purposes, including as a fertiliser and a substitute for sodium chloride in table salt. Its crystal structure gives it a high boiling point and an ability to conduct electricity, making it easy to extract as a demonstration of separation by electrolysis in schools.

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History

Potassium chloride salt was first discovered in 1807 during Sir Humphrey Davy's experiments into the possibility to separating the constituent elements in a compound using electrolysis. Despite its relatively late discovery, potassium chloride has been in common use by humans for millennia, being a constituent of potash and thereby used in the soap and textile industries.

Metallic Bonding

Potassium chloride is an ionic metal compound and as such, the elements it contains are subject to metallic bonding. The elements composing the potassium chloride salt form into a rigid crystalline structure of metal ions which are bound together through a mass sharing of electrons, sometimes referred to as an "electron sea."

Shape

Potassium chloride binds in a more complex version of the standard face-centred cubic structure whereby each face of the cube contains nine atoms in a three-by-three grid. The atoms are structured so one atom of potassium binds to a neighbouring chlorine along one axis, which then binds to another potassium and the cycle repeats. This face-centred cubic with a two-atom base structure is common in ionic metal compounds.

Size

Because of its cubic lattice structure, the lattice constants of potassium chloride are all equal at 630 picometers (0.0000000248 inches). A lattice constant is the distance over which a single group of atoms forming a unit or block within the lattice will repeat. The strength of the potassium chloride crystal structure is sufficient to allow these units to repeat thousands of times, creating large crystals.

Effects

The ionic metal crystal structure of potassium chloride allows the crystal to conduct electricity. It is far less efficient at conducting heat however, and has a thermal conductivity rating almost 20 per cent that of water. Because of the ionic structure of this salt, potassium chloride will dissolve readily in water.

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