A substance that cannot be reduced any further by chemical means. It is a pure chemical substance composed of atomswith the same atomic number (i.e. the same number of protonsin its nucleus). There are 92 naturally occurring elements on Earth, and all chemical matter consists of these elements (although a further 25 have been discovered as products of artificial nuclear reactions). Elements with atomic numbers 83 or higher are inherently unstable, and undergo radioactive decay. The list of elements is usually shown in the form of a Periodic Table, in order of their atomic number (see box at right, or click ther source link for a more detailed interactive Periodic Table).