Isotopes (from Greek words meaning "same position") refer to the different versions of a given element. They occupy the "same position" of an element in the Periodic Table, and are chemically identical. Each element is composed of a number of positively charged protons which are surrounded by negatively charged electrons. Hydrogen, being the simplest element, is composed of a single electron moving around a single proton. However, elements may possess various number of neutrons in the nucleus of the atom; the various number of neutrons determine the various number of isotopes (some elements have many isotopes, some have none.) Each isotope has a different atomic weight, since the nucleus may have different quantities of neutrons in the nucleus. In 1816, an English physician named William Prout (1785-1850) suggested that all atoms were constructed of multiple units of the hydrogen atom. However, his theory was disregarded; as atomic weights began to be determined, many elements had fractional weights instead of whole numbers as the theory required. Around the turn of last century chemists and physicists began experimenting with radioactivity, and began to realize that elements can have multiple forms, and therefore multiple weights, varying only by the number of neutrons, and it was this variation that caused the fractional weights in the atomic weight. Once the neutrons were considered, Prout's theory was vindicated, as each element and each of its isotopes all have integer weights.
"The New Intelligent Man's Guide to Science," I. Asimov, Basic Books, Inc., 1965, pg. 258.
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