Chemistry: Credo Reference

branch of science concerned with the properties, composition, and structure of substances and the changes they undergo when they combine or react under specified conditions.

Arrangement of the chemical elements in order of their atomic numbers in accordance with the periodic law first stated by chemist Dmitri Mendeleyev in 1869. The metallic transition elements are arranged in the middle of the table between groups II and III. Alkali metals are in group I, and alkaline-earth metals in Group II. Metalloids and nonmetals are found from groups III to VII, with the halogens in group VII and the noble gases (inert gases) collected into group 0. The elements in each group have the same number of valence electrons and accordingly have similar chemical properties. Elements in the same horizontal period have the same number of electron shells.

mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element, as contrasted with atomic mass, which is the mass of any individual isotope. Although the first atomic weights were calculated at the beginning of the 19th cent., it was not until the discovery of isotopes by F. Soddy (c.1913) that the atomic mass of many individual isotopes was determined, leading eventually to the adoption of the atomic mass unit as the standard unit of atomic weight. For many elements with two or more stable isotopes, atomic weight is now expressed as a variable interval with a lower and upper bound instead of a single number; in the case of hydrogen, for example, this is typically written as: [1.00784; 1.00811].

Branch of chemistry that deals with carbon compounds. Organic compounds form the chemical basis of life and are more abundant than inorganic compounds. In a typical organic compound, each carbon atom forms bonds covalently with each of its neighbouring carbon atoms in a chain or ring, and additionally with other atoms, commonly hydrogen, oxygen, nitrogen, or sulphur.

[Lat.,=charcoal], nonmetallic chemical element; symbol C; at. no. 6; interval in which at. wt. ranges 12.0096–12.0116; m.p. about 3,550degrees Celsius; graphite sublimes about 3,375degrees Celsius; b.p. 4,827degrees Celsius; sp. gr. 1.8–2.1 (amorphous), 1.9–2.3 (graphite), 3.15–3.53 (diamond); valence +2, +3, +4, or -4.

in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F. Soddy in explaining aspects of radioactivity; the first stable isotope (of neon) was discovered by J. J. Thomson. The nuclei of isotopes contain identical numbers of protons, equal to the atomic number of the atom, and thus represent the same chemical element, but do not have the same number of neutrons. Thus isotopes of a given element have identical chemical properties but slightly different physical properties and very different half-lives, if they are radioactive (see half-life). For most elements, both stable and radioactive isotopes are known.