An element can be defined as a substance
that cannot be broken down into a simpler one by means of a chemical
reaction. Elements (e.g. carbon and
oxygen) can often be combined (either by natural means or in a laboratory) to
form other substances known as compounds (e.g. carbon dioxide).
The factor that distinguishes one element
from another is the number of protons in its nucleus. The most basic element is
hydrogen, which has only one proton, hence its “atomic number” is 1.
To date, 118 different elements have been
identified, but many of the heavier elements do not occur naturally and have
only been created in laboratories.
Very heavy elements are unstable and
readily break apart, releasing particles and/or energy as they do so – these
are termed “radioactive”, such as uranium with an atomic number of 92 and
plutonium (atomic number 94).
The term “half-life” has been coined to
define the stability of an element, the factor in question being how long it
takes for half of a given quantity of the element to decay as the nucleus
breaks up and the substance changes into something that is more stable. Some
radioactive elements have half-lives of millions of years, whereas some
manufactured elements exist for only fractions of a second.
Various elements share certain features in
common and so can be grouped together. The standard method of organizing the
elements so that these shared features can be tabulated and easily recognised
is the Periodic Table (see illustration).
© John Welford
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