chemistry

Branch of science concerned with the study of the structure and composition of the different kinds of matter, the changes that matter may undergo, and the phenomena which occur in the course of these changes.

Organic chemistry is the branch of chemistry that deals with carbon compounds. Inorganic chemistry deals with the description, properties, reactions, and preparation of all the elements and their compounds, with the exception of carbon compounds. Physical chemistry is concerned with the quantitative explanation of chemical phenomena and reactions, and the measurement of data required for such explanations. This branch studies in particular the movement of molecules and the effects of temperature and pressure, often with regard to gases and liquids.

Atoms, molecules, and elements All matter can exist in three states: gas, liquid, or solid. It is composed of minute particles called atoms which are constantly moving. Atoms may be joined together by covalent bonds to form molecules, which may contain only two (such as oxygen and nitrogen) or up to hundreds of thousands (such as DNA and proteins) of atoms.

A specific type of atom, as identified by its chemical properties, represents a chemical element. All elements are classified in the periodic table of the elements, developed by English chemist John Newlands in 1863 and established by Russian chemist Dmitri Mendeleyev in 1869. At first, it classified elements according to their relative atomic masses. Those elements that resemble each other in general properties were found to bear a relation to one another by weight, and these were placed in groups or families. Certain anomalies in this system were later removed by classifying the elements according to their atomic numbers. The latter is equivalent to the positive charge on the nucleus of the atom. A specific type of atom, as identified by the number of protons it contains, represents a chemical element. All elements are classified in the periodic table of the elements.

Elements are divided into metals, which have lustre and conduct heat and electricity, and nonmetals, which usually lack these properties.

Compounds and mixtures A substance whose molecules contain different elements is a compound. Chemical compounds are produced by a chemical action that alters the arrangement of the atoms in the reacting molecules. Heat, light, vibration, catalytic action, radiation, or pressure, as well as moisture (for ionization), may be necessary to produce a chemical change. Examination and possible breakdown of compounds to determine their components is analysis, and the building up of compounds from their components is synthesis. When substances are brought together without changing their molecular structures they are said to be mixtures.

Formulas and equations Symbols are used to denote the elements. The symbol is usually the first letter or letters of the English or Latin name of the element - for example, C for carbon; Ca for calcium; Fe for iron (ferrum). These symbols represent one atom of the element; molecules containing more than one atom of an element are denoted by a subscript figure - for example, water is H2O. In some substances a group of atoms acts as a single entity, and these are enclosed in parentheses in the symbol - for example (NH4)2SO4 denotes ammonium sulphate. The symbolic representation of a molecule is known as a formula. A figure placed before a formula represents the number of molecules of a substance taking part in, or being produced by, a chemical reaction - for example, 2H2O indicates two molecules of water. Chemical reactions are expressed by means of equations as in:

NaCl + H2SO4 → NaHSO4 + HCl

This equation states the fact that sodium chloride (NaCl) on being treated with sulphuric acid (H2SO4) is converted into sodium bisulphate (sodium hydrogen sulphate, NaHSO4) and hydrogen chloride (HCl). See also chemical equation.

Alchemy Ancient civilizations were familiar with certain chemical processes - for example, extracting metals from their ores, and making alloys. The alchemists endeavoured to turn base (nonprecious) metals into gold, and chemistry evolved towards the end of the 17th century from the techniques and insights developed during alchemical experiments.

Landmarks in chemistry Robert Boyle defined elements as the simplest substances into which matter could be resolved. The alchemical doctrine of the four elements (earth, air, fire, and water) gradually lost its hold, and the theory that all combustible bodies contain a substance called phlogiston (a weightless ‘fire element’ generated during combustion) was discredited in the 18th century by the experimental work of Joseph Black, Antoine Lavoisier, and Joseph Priestley (who discovered the presence of oxygen in air).

Henry Cavendish discovered the composition of water, and John Dalton put forward the atomic theory, which ascribed a precise relative weight to the ‘simple atom’ characteristic of each element.

In the 19th century, synthetic dyes were discovered and their production became the foundation of the chemical industry. Numerous new elements and syntheses were discovered. Friedrich Wöhler's synthesis of urea in 1828 is often considered the birth of organic chemistry. Robert Brown discovered in 1827 the random thermal motion of molecules now known as Brownian motion. In 1848 Louis Pasteur made the discovery that certain molecules occur in two versions, which are mirror images of each other. Friedrich August Kekulé von Stradonitz realized in 1865 that carbon must be tetravalent and that alternating double and single bonds can account for the structure of benzene. Josiah W Gibbs stated some of the fundamental laws of thermodynamics, which became the foundation of physical chemistry.

In the 20th century, the new chemical disciplines such as polymer chemistry (leading to the production of a wide range of plastics, biochemistry, medicinal chemistry (see chemotherapy), quantum chemistry, and electrochemistry became increasingly important. The periodic table was expanded by the synthesis of new elements. New analytical methods have enabled chemists to investigate chemical processes at the level of single molecules. Many new materials developed by chemists replaced traditional materials in the production of consumer goods. Chemically produced fertilizers and s have had a huge impact on the food supply all over the world.