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atomic structure
The smallest particle of an element which can take part in a chemical reaction. An atom consists of a nucleus, containing positively charged protons and uncharged neutrons, which is surrounded by much lighter, negatively charged electrons. The number of protons is equal to the number of electrons so that, overall, an atom is electrically neutral.

Atoms are inconceivably small. A normal drop of water contains about 6,000 quintillion (a 6 × 1021) atoms. The diameter of an atom consisting of a nucleus and orbiting electrons is approximately one hundred millionth of a centimeter (10-8 cm). The diameter of the nucleus is approximately ten trillionths of a centimeter (1 to 5 × 10-13 cm). The nucleus is therefore 100,000 times smaller than the surrounding sheath of orbiting electrons.

History of atomic theory

From the days of Democritus onward the atomic idea in a vague form appeared at intervals. Robert Boyle and Isaac Newton used it in their physical speculations, but at the beginning of the 19th century, John Dalton made it into a definite quantitative theory, based on the numerical facts of chemical combination.

Antoine Lavoisier and others had proved that a chemical compound, however obtained, is always made up of the same amount of the same constituent parts – water, for instance, always consists of one part of hydrogen combined with eight of oxygen, giving 8 as the "combining weight" of oxygen. Nevertheless, some chemists, among them Berthollet, did not believe in this constancy. But John Dalton saw that the properties of gases were best explained by atoms, and then pointed out that, on this theory, the combining weights in chemical combination gave the relative weights of the atoms also. He drew up a list of 20 such atomic weights (see relative atomic mass). This formulation was too simple; Dalton thought that, if only one compound of two elements was known, it was fair to assume it was formed atom for atom. This is not always true, indeed it put Dalton wrong about water.

Joseph Gay-Lussac showed that gases combine in volumes that bear simple ratios to each other, and Amedeo Avogadro pointed out that this must apply also to the numbers of their combining mass. In 1858 Stanislao Cannizzaro saw that it was necessary to distinguish between the chemical atom, the smallest part of matter which can enter into chemical action, and the physical molecule, the smallest particle which can exist in the free state. The simplest form of Avogadro's hypothesis is to suppose that equal volumes of gases at the same temperature and pressure contain the same number of molecules, a result which follows also from kinetic theory of gases. In the formation of water from its elements we have two volumes of hydrogen combining with one of oxygen to form two of water vapor. The simplest theory which will explain these facts is to suppose that each molecule of hydrogen or oxygen contains two atoms, and that

2H2 + O2 right arrow 2H2O

i.e., two atoms of hydrogen combine with one of oxygen to form one molecule of water. Oxygen is therefore said to be a divalent element. The concept of valence has been much used in chemical theory. From the considerations given above it follows that the atomic weight of oxygen is not 8 but 16. Thus Dalton's combining weights need to be considered in the light of other experiments before we can assign to the elements their true atomic weights. This was first done systematically by Cannizzaro.

A connection between atomic weights and physical and chemical properties was sought by several chemists, the most successful being the Russian Dmitri Mendeleyev. On arranging the elements in order of ascending atomic weights, a periodicity appeared, each eighth element having somewhat similar properties. The resulting periodic table gave a means of assigning correct atomic weights to elements of doubtful valency. Blanks in the table were filled hypothetically by Mendeléeff, who thus predicted the existence and properties of unknown elements, some of which were afterwards discovered. As measured chemically, many elements (but not all) have atomic weights approaching whole numbers. This suggested to Prout and others that all elements were composed of hydrogen, or at all events of some common basis. But this idea was beyond the theoretical and experimental powers of the time to test.

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Sources: European Nuclear Society website (first section); A Shorter History of Science by W. C. Dampier, Cambridge University Press, 1945 (history section).