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Isotopes
Isotopes
Two or more atoms that have the same number of protons in their nucleus, but that have a different atomic mass because the number of neutrons can vary; for example, carbon-12 and carbon-14. Fluorine has only one isotope,
but elements such as tin have ten. Most elements have at least two isotopes. Some isotopes are radioactive.
K
Krypton (Kr)
Element 36 on the periodic table. One of the noble gases from group 8.
It is an inert gas and forms very few compounds.
Krypton is heavier than air, colourless, odourless and tasteless. Krypton was discovered in 1898 by the British chemists Sir William Ramsay and Morris W. Travers.
Krypton is used in some types of fluorescent lamps. When a current of electricity is passed through a glass tube containing krypton at low pressure, it gives out a bluish- white light.
L
Lanthanum (La)
Element 57 on the periodic table. A member of the rare-earth metals and one of the lanthanides.
Lanthanum is a soft, silvery- white metal that is easy to bend (malleable). It was discovered in 1839 by Carl Gustaf Mosander.
Its name comes from a Greek word meaning ‘to be concealed’ because it was particularly difficult to separate from its compounds. Lanthanum oxide is used in high- quality glass lenses.
Lanthanide
An element belonging to the lanthanide series – 17 similar metallic elements starting with scandium (21) and yttrium (39) and including the elements from lanthanum (57) to lutetium (71) in period 6 on the periodic table. The lanthanides are also called rare-earth metals or rare-earth elements and they are transition metals.
Lavoisier, Antoine- Laurent
A French chemist who, in 1789, published the first list of elements, following the definition of an element by Robert Boyle.
Lavoisier’s list contains some substances (lime, alumina and silica) that we now know as compounds because he did not have ways to separate them. He also included light and heat (known as caloric) on his list.
(See also: Hydrogen; Nitrogen; Oxygen.)
Lawrencium (Lr)
Element 103 on the periodic table. An artificial and radioactive element of the actinide series.
It was produced in 1961 by Albert Ghiorso, T. Sikkeland, A. E. Larsh and R. M. Latimer. Lawrencium is a transuranium element.
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Lead (Pb)
Element 82 on the periodic table. A soft, silvery-white metal in group 4 (the carbon group).
Lead is very dense, soft and easily shaped (malleable). It is also a poor conductor of electricity. When lead is exposed to air, it quickly develops a coating that turns it dull brown.