Page 42 - Curriculum Visions Dynamic Book

Key facts...
Name: chromium
Symbol: Cr
Atomic number: 24
Atomic weight: 51.99
Position in Periodic Table: transition metal, group
(6) (chromium group); period 4
State at room temperature: solid
Colour: steel-grey
Density of solid: 7.2 g/cc
Melting point: 1,890°C
Boiling point: 2,670°C
Origin of name: from the Greek word chroma,
meaning colour, a reflection of the wide variety
of colours found in chromium compounds. Shell pattern of electrons: 2–8–13–1
Chromium (Cr)
Element 24. Chromium, a steel-grey metal, is a transition metal in the Periodic Table.
Chromium compounds are highly coloured.They give colour to many natural substances such as emerald (green) and ruby (red) and add red
to the paint called Siberian red lead (PbCrO4, crocoite). Chromium metal does not react with water at room temperature.
Discovery
Chrome plating is used to protect other metals.
Biology
Chromium is an essential trace element. However, in higher quantities chromium and chromium compounds are toxic.
Chromium was discovered in 1797 by the French chemist Nicolas-LouisVauquelin.
Its possible existence was first noticed because of the colour that some compounds such as Siberian red lead had, which clearly had nothing to do with their lead content. Vauquelin discovered that the green colour of an emerald is also due to chromium.
Technology
Chromium is very hard and can be polished. It is used extensively in plating other materials, especially steel (on cars) and brass (on taps). 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, for example, is an alloy of iron and chromium. Adding it to glass makes the glass emerald green. Chromium is also useful as a catalyst, making a variety of reactions take place faster and more efficiently.
Geology
Chromium is mainly found in chromite ore Fe(CrO2)2.
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For more on chromium, see Volume 4: Iron, Chromium and Manganese in the Elements set.