<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" was coined by ancient Greek philosophers . However, these ideas were founded in philosophical and theological reasoning rather than evidence and experimentation . As a result, their views on what atoms look like and how they behave were incorrect . They also could not convince everybody, so atomism was but one of a number of competing theories on the nature of matter . It was not until the 19th century that the idea was embraced and refined by scientists, when the blossoming science of chemistry produced discoveries that only the concept of atoms could explain . </P> <P> In the early 1800s, John Dalton used the concept of atoms to explain why elements always react in ratios of small whole numbers (the law of multiple proportions). For instance, there are two types of tin oxide: one is 88.1% tin and 11.9% oxygen and the other is 78.7% tin and 21.3% oxygen (tin (II) oxide and tin dioxide respectively). This means that 100g of tin will combine either with 13.5 g or 27g of oxygen . 13.5 and 27 form a ratio of 1: 2, a ratio of small whole numbers . This common pattern in chemistry suggested to Dalton that elements react in whole number multiples of discrete units--in other words, atoms . In the case of tin oxides, one tin atom will combine with either one or two oxygen atoms . </P>

When was the idea of the atom first introduced