ATOMIC NATURE OF MATTERDOE-HDBK-1019/1-93Atomic and Nuclear PhysicsIn 1661 the English chemist Robert Boyle published the modern criterion for an element. Hedefined an element to be a basic substance that cannot be broken down into any simplersubstance after it is isolated from a compound, but can be combined with other elements to formcompounds. To date, 105 different elements have been confirmed to exist, and researchers claimto have discovered three additional elements. Of the 105 confirmed elements, 90 exist in natureand 15 are man-made.Another basic concept of matter that the Greeks debated was whether matter was continuous ordiscrete. That is, whether matter could be continuously divided and subdivided into ever smallerparticles or whether eventually an indivisible particle would be encountered. Democritus in about450 B.C. argued that substances were ultimately composed of small, indivisible particles that helabeled atoms. He further suggested that different substances were composed of different atomsor combinations of atoms, and that one substance could be converted into another by rearrangingthe atoms. It was impossible to conclusively prove or disprove this proposal for more than 2000years.The modern proof for the atomic nature of matter was first proposed by the English chemist JohnDalton in 1803. Dalton stated that each chemical element possesses a particular kind of atom,and any quantity of the element is made up of identical atoms of this kind. What distinguishesone element from another element is the kind of atom of which it consists, and the basic physicaldifference between kinds of atoms is their weight.SubatomicParticlesFor almost 100 years after Dalton established the atomic nature of atoms, it was consideredimpossible to divide the atom into even smaller parts. All of the results of chemical experimentsduring this time indicated that the atom was indivisible. Eventually, experimentation intoelectricity and radioactivity indicated that particles of matter smaller than the atom did indeedexist. In 1906, J. J. Thompson won the Nobel Prize in physics for establishing the existence ofelectrons. Electrons are negatively-charged particles that have 1/1835 the mass of the hydrogenatom. Soon after the discovery of electrons, protons were discovered. Protons are relativelylarge particles that have almost the same mass as a hydrogen atom and a positive charge equalin magnitude (but opposite in sign) to that of the electron. The third subatomic particle to bediscovered, the neutron, was not found until 1932. The neutron has almost the same mass as theproton, but it is electrically neutral.NP-01Page 2Rev. 0
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