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Chemistry is basically the study of all the different kinds of materials, and of the ways they can be combined or separated, particularly by simple low energy methods such as heating, cooling, mixing, sorting, and passage of electric currents. A homogeneous material may be called a substance. Many materials occurring in nature can easily be seen to be mixtures of several different kinds of materials, though their separation can often be difficult. A substance, that cannot be separated into simpler materials, is an element, otherwise it is a compound formed by combining elements. It is often difficult to remove all traces of impurities. Some mixtures, such as metallic alloys can be so thoroughly mixed as to be difficult to distinguish from a pure compound. The elucidation of which materials were truly elementary and which compound was not fully resolved until the end of the nineteenth century.
The classification of natural materials into mineral, vegetable and animal is of course prehistoric. Aristotle (−384 - −322) spoke formally of a 'chain of being' from minerals, through plants to animals and humans, though this was a recognition of increasing complexity rather than of any evolutionary relationship.
The early Greek philosopher Zeno of Elea, (c.−490 - c.−430) as part of his investigations into the concept of infinity, seems to have been the first to raise the question of whether matter (or indeed space and time), could be infinitely subdivided into, smaller and smaller parts. This led Leucippus (c.−480 - c.−420) and Democritus (c.−460 - c.−370) to develop the beginnings of a theory of matter as being composed of small particles, which they called atoms, meaning 'not cuttable'. Their ideas were adopted by Epicurus (−341 - −271). The Roman author Titus Lucretius Carus (c.−99 - −55) expounded the ideas in his epic poem De Rerum Natura (On The Nature of Things).
However the more popular idea was that espoused by Empedocles & (c.−494 - c.−434) and Plato among others (and similar ideas were known in India and China) of all matter being composed of just four elements & &, earth, water, air and fire (and sometimes a fifth element 'quintessence' or 'ether'). In modern terms these are not 'elements' at all, but rather states of matter: earth being solid, water liquid, air gaseous, and fire being a process of change by chemical reaction. These four 'elements', with elaborations, were followed by the 'alchemists' throughout the middle ages, and into the renaissance, and are still popular with all sorts of mystical groups.
Seven metals (materials that are malleable, shiny and conductive of heat) have been known since antiquity. A Short History of Metals by Alan W. Cramb lists the approximate dates at which they were first worked in pure form as: Gold −6000, Copper −4200, Silver −4000, Lead −3500, Tin −1750, Iron −1500, and Mercury (also known as Quicksilver, which is liquid under normal conditions) −750. This was before any theories of chemical composition were proposed. These seven are all now known to be elements. The seven metals were fancifully associated with the seven moving celestial bodies visible to the unaided eye, and were often denoted by the same names and symbols: Sun, Venus, Moon, Saturn, Jupiter, Mars, Mercury; and also with the seven days of the week. This symbolism was used by alchemists throughout the middle ages and is still observed by astrologers.
The study of materials as they are found in nature is mineralogy. One of the strongest hints that matter might be formed of atoms is provided by the fact that many natural materials occur in the form of crystals, which suggests that they are built up from small components that fit together in a regular lattice. The study of crystallography & & & & involves quite complex geometry.
There is no clear dividing line in history between alchemy and modern chemistry. A more scientific approach was very gradually developed, for instance from Philip von Hohenheim, known as 'Paracelsus' & & & & (1493 - 1541), through Jan Baptista van Helmont & & (1579 - 1644) to Robert Boyle & & & & & & (1627 - 1691). Much of the work however was handicapped by a mistaken theory of combustion propounded by Johann J. Becher (1625 - 1682) and Georg Ernst Stahl & (1660 - 1743) that in all flammable materials there is present a substance, called by Becher 'fatty earth' and by Stahl phlogiston, which is supposedly given off in burning.
Real progress in chemistry began to be made in the 1770s when, by more accurate gathering of all the gaseous components and careful weighings, a number of rival chemists, including Joseph Black & (1728 - 1799), Henry Cavendish & & (1731 - 1810), Joseph Priestley & & (1733 - 1804), Carl Wilhelm Scheele & & & (1742 - 1786) and Antoine Laurent Lavoisier & & & & & (1743 - 1794) isolated oxygen and at last understood combustion in air as combination of other elements with oxygen. Air was shown not to be an element but to be a mixture of oxygen and nitrogen with smaller quantities of other gases. Water was shown to be not an element but a combination of hydrogen with oxygen.
Before the 18th century electricity was known only in the form of static charge produced by rubbing of one substance such as glass or amber (whose Greek name is 'electron') with another. It was discovered that quite high charges could be stored in a device known as a Leyden jar & & & (a capacitor). The experiments of Luigi Galvani & & (1737 - 1798) and Alessandro Volta & & (1745 - 1827) led to invention of electric batteries (or 'piles'), which generate electric current by stacking discs of different metals, such as silver and zinc, separated by brine-soaked pads.
It was Humphry Davy & & & & & (1778 - 1829) who realised that the electricity was being produced by a chemical reaction between these substances. He began to investigate the converse process of electrolysis in which electric current is passed through a liquid material, resulting in gases being emitted, or material being deposited on or eaten away from the electrical connectors (known as electrodes, either positive anode or negative cathode). By this means new elements such as potassium, sodium and calcium were isolated from their compounds for the first time. Compounds such as sodium chloride (common salt) were seen as consisting of positive and negative charged particles called ions, held together by electrical attraction. Davy's work was carried on and quantified by his assistant and successor at the Royal Institution, Michael Faraday (1791 - 1867). The terms shown in bold were introduced at the suggestion of William Whewell (1794 - 1866), who investigated the history of scientific method and also seems to have been the first to use the term scientist.
John Dalton & & & (1766 - 1844), taking account of these new discoveries in chemistry, revived the atomic theory in modern form with A New System of Chemical Philosophy 1808 (revised 1810, 1827). However our modern system of chemical notation owes more to the work of the Swedish chemist Jons Jacob Berzelius & & & & & & (1779 - 1848) who measured accurate "atomic weights" & & & &.
About the same time, 1808, Joseph Louis Gay-Lussac (1778 - 1850) found that gases combine in simple proportions by volume. This meant that for most materials in gaseous form the atoms tend to stick together in clumps called molecules. The useful distinction between atoms and molecules was due largely to Amedeo Avogadro & & (1776 - 1856) in 1811. However his ideas were not immediately accepted and confusion reigned until the first International Chemical Congress (1860) at which Stanislao Cannizzaro (1826 - 1910) revived his ideas.
Avogadro also postulated the principle now known by his name, that equal volumes of all gases at the same temperature and pressure contain the same number of molecules. From these considerations it followed that the chemical formulas for hydrogen, oxygen and water were not H, O and HO (as Dalton expressed it) but H2, O2 and H2O. However it was not known what the forces could be that held these molecules together in chemical bonds. This was not resolved until the development of the valence bond theory (1914) and quantum chemistry (see below).
The actual sizes of air molecules were first estimated by Johann Joseph Loschmidt & & (1821 - 1895) in 1865: On the Size of the Air Molecules. This enabled him to estimate the so-called "Avogadro number"; its currently accepted value is 6·022.10^23 per mole (a "mole" being the molecular weight of a substance in grams). He was also the first to draw formulas using double and triple bonds.
Gradually, as more elements were isolated, regularities began to be observed in the properties of the elements in relation to their atomic weights. Work was done on this by J. W. Dobereiner & (1780 - 1849) and others. This was systematised in 1869 by Dmitri Mendeleev & & & & (1834 - 1907) in the form of a periodic table & & of the 65 elements then known. This showed that there were gaps and predicted the existence of other elements to fill these gaps, and their likely properties. This led to the discovery of gallium (1875), scandium (1886) and germanium (1879). About the same time, Julius Lothar Meyer & & & (1830 - 1895) found similar regularities by plotting atomic weight against atomic volume.
Isaac Newton & & famously studied the splitting of sunlight into its rainbow spectrum of colours by its passage through a prism of glass. From 1802 onwards various observers noted dark and bright lines that appeared in the spectrum of sunlight. Joseph von Fraunhofer (1787 - 1826) designed achromatic telescope lenses and began to study these lines. The philosopher August Comte (1798-1857) expressed the quite reasonable view, in 1835, that we could probably never know the composition of the stars. Robert Bunsen & (1811 - 1899) and Gustav Kirchhoff &(1824 - 1887) however showed that each chemical element absorbs and emits its own characteristic combination of wavelengths, thus enabling the existence of particular chemicals in the sun and stars to be identified by what came to be known as spectral analysis &. The element helium & was famously named from its discovery in the sun's spectrum.
The discovery of X-rays by Wilhelm Roentgen & & (1845 - 1923) in 1895, led eventually to the important process of X-ray crystallography & & & &, which was developed by Max von Laue & & (1879 - 1960), William Henry Bragg (1862 - 1942) and William Laurence Bragg & & (1890 - 1971) and others. X-ray crystallography is made possible since X-rays are a form of light of short wavelength, comparable to the sizes of atoms. The methods were first applied (1914) to determine the three-dimensional structure of inorganic materials: salt, diamond, fluorspar, zincblende, iron pyrites, calcite, dolomite, copper and in the 1920s other metals, alloys and silicates. It later proved important in analysing complex organic molecules (see below).
The distinction between materials deriving from animal and vegetable sources as being organic while others are inorganic is a natural one to make, though the processes of break-down and fossilisation of animal and vegetable remains and the effects of great heat and pressure, leading for instance to the formation of coal and chalk complicate the issue. In modern chemistry the distinction is seen to be one between polar compounds held together by electrical attraction, and thus separable by electrolysis, and nonpolar compounds, in which the bonds are predominantly covalent.
It was widely held that living or organic matter was fundamentally different from inorganic or dead matter and that some vital force was needed. The first dent in this belief was made by Friedrich Wohler & (1800 - 1882) who in 1828 synthesised urea, formula (NH2)2CO, and showed it to be an isomer of ammonium cyanate, formula NH3OCN, i.e. formed of the same atoms arranged in a different structure.
Studies by Justus Freiherr von Liebig & (1803 - 1873), Animal Chemistry 1842, Organic Chemistry 1843, Agricultural Chemistry 1855, and Louis Pasteur (1822 - 1895), revealed commonalities between the chemical principles of the organic and inorganic worlds. In 1897 Eduard Buchner (1860 - 1917) demonstrated fermentation of alcohol in the absence of cells. Wilhelm Kuhne (1837 - 1900) who studied the chemistry of digestive processes introduced the term enzyme in 1876 to describe organic substances which activate chemical changes.
Many other products from organic sources were subsequently sythesised, and new organic compounds produced. William Henry Perkin & & (1838 - 1907) in 1856 made the first aniline purple dye, mauveine, from coal-tar extracts. He set up a business to manufacture the dye, which was the beginning of the pharmaceutical industry. He also synthesised alizarin (1868) the basis of natural red dye, and coumarin (1873) used in perfumes. By the end of the century dye companies had expanded into major chemical industries (Hoechst, BASF, Bayer, Agfa, Ciba, Geigy).
Aspirin & & (acetyl salicylic acid C9H8O4) was synthesised by Bayer in 1897. Ammonia (NH3) was synthesised by Fritz Haber & & & & (1868 - 1934) in a small way in 1904, this was later developed into the Haber-Bosch process.
Friedrich August Kekule & (1829 - 1896) in 1865 elucidated the structure of benzene (C6H6) as a ring of carbon atoms. This opened up the study of 'aromatic' as opposed to 'aliphatic' chemistry of carbon compounds. Many chemicals of organic origin involve long carbon chains, often with cyclic links. In diagrammatic formulas the C's and the H's directly attached to them are often omitted and the chain is reduced to straight lines representing the bonds between the carbon atoms, so that the benzene ring & is shown as a hexagon, sometimes with alternating single and double bonds, though this is an over-simplification.
Emil Fischer & & & (1852 - 1919), discovered the three sugars, glucose, fructose and mannose in 1888, and by 1890 was the first to synthesize them, starting from glycerol. In plants glucose is manufactured from carbon dioxide and water by photosynthesis: 6 CO2 + 6 H2O = C6H12O6 + 6 O2 with chlorophyll as catalyst. Fischer in 1894 also elucidated the way enzymes work by a 'lock and key' mechanism; each enzyme being structured uniquely to recognise one particular chemical compound, even one particular form of sugar differing only by its spatial structure from others.
Richard Willstatter & (1872 - 1942), in 1913 analysed the structure of chlorophyll & & & as a porphyrin ring, "co-ordinated" to a central atom. This is very similar in structure to the heme group found in hemoglobin, except that in heme the central atom is iron, whereas in chlorophyll it is magnesium.
Polymers and Plastics: polyethylene synthesised in 1933.
Dorothy Hodgkin & & & & (1910 - 1994), used X-ray crystallography to solve the structures of organic compounds: cholesterol (1937), vitamin B12 (1945), penicillin (1954) and insulin (1969). Max Perutz & & (1914 - 2002), determined the structure of hemoglobin (1959).
Francis Crick & & & (1916 - 2004), Maurice Wilkins (1916 - 2004), Rosalind Franklin (1920 - 1958), and James Watson & & & & & (1928 - ), used X-ray crystallography and other data to deduce the structure of DNA: The Double Helix & 1968, The Molecular Biology of the Gene, Discovery of Structure of DNA, Of Molecules and Men 1966.
Other syntheses: fullerenes discovered in 1985.
The work of the physicists J. J. Thomson who discovered the negatively charged electron in 1895, and of Ernest Rutherford who in 1911 found the atom to consist of a central, positively charged, heavy nucleus surrounded by electrons, provided a model by means of which the regularities observed in the periodic table could be explained by breaking the previously 'uncuttable' atoms into even more elementary components.
The Bohr model was its successor, first proposed in 1913, incorporating non-classical notions of physics, namely the quantization of orbital angular momentum.
The neutron was not discovered until 1932, by James Chadwick, by whch time, quantum mechanics was already established, thanks to Heisenberg, Schrödinger, Born and others.
The modern model of atoms and molecules is quantum chemistry & & which uses quantum theory to mathematically describe the behavior of matter at the molecular scale, but in practice only the simplest chemical systems can be accurately calculated, and approximations must be made. The important implications of the theory can be understood in simpler terms, namely atomic orbitals & & & X X X. (X denotes animation). Roughly speaking electrons are represented as "clouds", arranged round the atomic nucleus, of various energy levels and shapes determined by three quantum numbers, which take only integral values. When two atoms close together the orbital probabilities can reinforce each other or cancel each other out. &.
The existence of electron shells was first observed experimentally in Charles Barkla's and Henry Moseley's X-ray absorption studies. Gilbert Lewis & (1875 - 1946) The Atom and the Molecule (1916) developed the modern valence bond theory of chemical structure, together with Irving Langmuir (1881 - 1957), The Arrangement of Electrons in Atoms and Molecules 1919. Linus Pauling later generalized and extended the theory while applying insights from quantum mechanics. In 1931 Pauling explained the structure of the benzene ring in terms of what he called 'resonance' using quantum theory.
[This section is incomplete, since it overlaps with information that will appear in the Physics section, which is being prepared.]
General links on chemistry
History of Elements an interactive periodic table
Classic Papers in History of Chemistry
100 Distinguished European Chemists
Chemical Heritage Foundation
Molecule of the Month
History of the Molecule