A chemical element is defined as a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Oxygen, for example, has 8 protons in its nucleus so it has an atomic number of 8. Oxygen atoms that have a different number of neutrons are isotopes, but are still the same element. The atomic mass of an atom is the combination of the number of neutrons and protons. For example carbon has an atomic mass of 12, since it has 6 protons and 6 neutrons.
It was previously believed that chemical elements were the smallest unit of a substance. It is now known that this can be broken down into protons neutrons and electrons.
Elements that have an atomic number's 83 and above are unstable, and thus are radioactive.
History of the Chemical Elements:
The term element was first used by the Greek philosopher Plato. There were only four elements that Plato recognized, earth, wind, water and fire. He deemed that elements were the smallest unit a substance was able to be broken down into.
It wasn't until 1661 that Robert Boyle published his book, The Sceptical Chymist, in which he disproved the idea of only four elements. Then in 1869 Dmitri Mendeleev published his Periodic Table of the Elements which not only listed 66 different elements, but had them sorted by both characteristics and atomic weights. While he knew that he did not have all the elements to complete his chart, he skipped spaces that would correspond to these unknown elements. Eventually these gaps were filled in when later scientists discovered more elements.
The Current Day Status of the Chemical Elements:
As of October 2009 IUPAC, the International Union of Pure and Applied Chemistry, currently recognizes 112 elements, 94 of which occur naturally on earth. Elements 113 to 116 and element 118 have been claimed to have been discovered by researchers and have been published in peer reviewed journals.