<P> The electrons of an atom are attracted to the protons in an atomic nucleus by this electromagnetic force . The protons and neutrons in the nucleus are attracted to each other by a different force, the nuclear force, which is usually stronger than the electromagnetic force repelling the positively charged protons from one another . Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force, and nucleons can be ejected from the nucleus, leaving behind a different element: nuclear decay resulting in nuclear transmutation . </P> <P> The number of protons in the nucleus defines to what chemical element the atom belongs: for example, all copper atoms contain 29 protons . The number of neutrons defines the isotope of the element . The number of electrons influences the magnetic properties of an atom . Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules . The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature and is the subject of the discipline of chemistry . </P> <P> The idea that matter is made up of discrete units is a very old idea, appearing in many ancient cultures such as Greece and India . The word "atom" (Greek: ἄτομος; atomos), meaning "uncuttable", was coined by the ancient Greek philosophers Leucippus and his pupil Democritus (c. 460--c. 370 BC). Democritus taught that atoms were infinite in number, uncreated, and eternal, and that the qualities of an object result from the kind of atoms that compose it . Democritus's atomism was refined and elaborated by the later philosopher Epicurus (341--270 BC). During the Early Middle Ages, atomism was mostly forgotten in western Europe, but survived among some groups of Islamic philosophers . During the twelfth century, atomism became known again in western Europe through references to it in the newly - rediscovered writings of Aristotle . </P> <P> In the fourteenth century, the rediscovery of major works describing atomist teachings, including Lucretius's De rerum natura and Diogenes Laërtius's Lives and Opinions of Eminent Philosophers, led to increased scholarly attention on the subject . Nonetheless, because atomism was associated with the philosophy of Epicureanism, which contradicted orthodox Christian teachings, belief in atoms was not considered acceptable . The French Catholic priest Pierre Gassendi (1592--1655) revived Epicurean atomism with modifications, arguing that atoms were created by God and, though extremely numerous, are not infinite . Gassendi's modified theory of atoms was popularized in France by the physician François Bernier (1620--1688) and in England by the natural philosopher Walter Charleton (1619--1707). The chemist Robert Boyle (1627--1691) and the physicist Isaac Newton (1642--1727) both defended atomism and, by the end of the seventeenth century, it had become accepted by portions of the scientific community . </P>

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