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Chromium (Cr)
 Chromium – A chromium-plated wheel.
Chromium (Cr)
Element 24 on the periodic table. A steel-grey metal and one of the transition metals.
It is very hard and can be polished. It resists corrosion, does not tarnish in air or corrode in water, and can be used in an alloy to increase hardness and to resist corrosion. Stainless steel is an alloy of iron and chromium.
Chromium was discovered in 1797 by the French chemist Nicolas-Louis Vauquelin.
Chromium compounds are highly coloured. They give their colour to many natural substances such as emerald and ruby.
Cobalt (Co)
Element 27 on the periodic table. One of the transition metals. Although the distinctive blue colour of cobalt compounds had been used since ancient times, it was only in 1735 that the Swedish
chemist Georg Brandt discovered its source to be the element cobalt.
Cobalt is also an essential part of blood cells as vitamin B12.
Cobalt is a scarce metal usually found as a trace element among ores of other elements such as iron and zinc. It is normally recovered as a by-product of refining these metals. Cobalt is very reactive and, if finely divided, it will ignite spontaneously.
Cobalt is magnetic and has a high melting point, so it can be used where magnetism is required in a hot environment. Radioactive isotopes of cobalt are used for medical purposes and also to look for flaws in materials.
The main use of cobalt is as
a metal in alloys, both for high temperature permanent magnets and for making very hard steels. Cobalt phosphate is a blue colouring agent in ceramics and glass. (See also: Catalyst.)
Coinage metals
The metals copper, gold and silver.
Combustion
A chemical reaction between oxygen and another substance. Some combustion reactions are slow, such as the combustion of the sugars we eat that provide energy for our bodies. If the combustion results in a flame, it is called burning.
Compound
A chemical consisting of two or more elements bonded together. (See also: Bond; Covalent bond; Oxide; Reduce, reduction; Salt.)
(For more on compounds see: Antimony; Barium; Bismuth; Bromine; Calcium; Carbon; Chlorine; Chromium; Cobalt; Gallium; Hydrogen; Iodine; Iridium; Magnesium; Silicon; Sodium; Transition elements; Vanadium.)
 Combustion – Combustion of steel in oxygen.
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