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Element
Hydrogen Helium Oxygen Carbon Neon Nitrogen Silicon
Iron Sulphur Argon Magnesium Aluminium Calcium Sodium Phosphorus Potassium
Actinide
% (in the universe)
87 12
0.06
0.03
0.02 0.008 0.003 0.002 0.002 0.0004 0.0003 0.0002 0.0001 0.0001 0.00003 0.000007
An element belonging to the actinide series – 15 similar radioactive elements running from actinium (atomic number 89)
and lawrencium (atomic number 103) in period 7 on the periodic table. The actinides are transition metals. The most well-known members of the series are uranium and plutonium, which are used
in nuclear reactors to produce electrical energy.
All of the elements in the group are radioactive, and those from americium (atomic number 95) onwards are made artificially in the laboratory and don’t appear
in nature.
Actinium (Ac)
Element 89 on the periodic table. A radioactive chemical element belonging to the actinide series.
It was discovered in 1899 by André-Louis Debierne. This rare, silvery-white metal glows blue in the dark. Actinium is one of the
Alkaline-earth metals
A
Abundance of the elements
decay products of uranium (see: Radioactive decay).
Alchemy
A medieval practice of trying to turn certain ‘imperfect’ metals, such as lead, into ‘perfect’ metals, such as gold. This was called transmutation and ways of doing
it were shrouded in secrecy. Of course, the practice did not work and alchemy gave way to scientific chemistry in the 18th century.
Alkali metals
The elements in group 1 on the periodic table.
They are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr).
The alkali metals have a silvery lustre. They can easily be made into new shapes (are ductile), and
 Alkali metals – Sodium is the most important alkali metal. It is highly reactive
in air and even reacts under water. This is a piece of sodium metal in alcohol. Notice how it tarnishes as soon as it is cut. These pictures were taken about 30 seconds apart.
they conduct electricity and heat well. They have low melting points.
All of them are extremely reactive and interact violently with water. Most common non-metallic substances (non-metals) react with the alkali metals.
Alkaline-earth metals
The elements in group 2 on the periodic table.
They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra). The term ‘alkaline- earths’ comes from the fact that metal oxides are soluble in water and cannot be burned. They were traditionally called ‘earths’. The ‘earths’ that have properties similar to alkalis (soda ash and potash) were thus called alkaline earths. Quicklime (calcium oxide) is a common example.
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