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Hydrogen (H)
Element 1 on the periodic table. The most common element in the universe. It has properties like those elements in groups 1 and 8. It usually appears in group 1 but is often placed on its own and is also thought of as being in group 0.
Hydrogen accounts for about 87% of all matter in the universe. Because it is so light, hydrogen is rare in the Earth’s atmosphere. It is, however, abundant in compounds – for example, it makes up just over a tenth of the mass of seawater.
Hydrogen is a colourless, odourless, tasteless, flammable gas. It is the simplest of all elements, having just one proton in its nucleus and one electron.
Hydrogen was discovered by the English chemist Henry Cavendish
in 1766. At that time it was called inflammable air or phlogiston. The name hydrogen was given by French chemist Antoine-Laurent Lavoisier from the Greek meaning ‘maker of water’.
Hydrogen gas goes into many chemicals, including ammonia (which is then made into fertilisers). It is also used in some food, such as in margarine.
Inert
Unreactive. The noble gases and nitrogen are examples of inert gases.
Insoluble
A substance that will not dissolve. Many substances will not dissolve
in one solvent, but will dissolve in another. Oil is insoluble in water but is soluble in many organic solvents.
 Iodine – Iodine is a purple gas and solid.
Iodine (I)
I
Indium (In)
Element 49 on the periodic table. A rare silvery-white metal belonging to group 3 (the boron group).
Discovered in 1863 by Ferdinand Reich and Theodor Richter, it is very soft and can easily be made to change shape (see: Malleable). Pure indium ‘screams’ when it is bent.
Indium is used in making electronic components.
 Iodine – The seaweed kelp is a source of natural iodine.
Iodine (I)
Element 53 on the periodic table. A non-metallic element and one of the halogens (group 7 elements).
Iodine does not occur naturally. Its compounds can be processed to produce pure dark-violet crystals. At room temperature solid iodine turns into a vapour (sublimes)
to make an unpleasantly irritating violet vapour. It is used in medicine (a dilute solution of iodine in alcohol is called
tincture of iodine and works
as an antiseptic), in
photography and in dyes.
Iodine is one of the
essential nutrients in the
body. To ensure that people
get enough of it, iodine is
added to most table salt.
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