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The United Nations has proclaimed 2019 the International Year of the Periodic Table of Chemical Elements (IYPT2019). The same year also marks the 150th anniversary of the publication of Russian chemist Dmitri Mendeleev’s periodic table, containing…

Either the taking up of energy from radiation by the medium through which radiation is passing, or the taking up of matter in bulk by other matter. A simple example of the absorption of energy is how sunlight warms a tree's leaves (see…

A geminal diether (R1 = H). Ketals, considered a subclass of acetals, are also geminal diethers (R1 = C, aliphatic or aromatic). Acetals are (1) independent structural units or a part of certain biological and commercial polymers, (2) blocking or…

An organic compound with the formula C2H2 or HCCH. The first member of the alkynes, acetylene is a gas with a narrow liquid range; the triple point is −81°C (−114°F). The heat of formation (ΔH°f ) is +227 kilojoules/mole, and acetylene is the most…

One of an important class of reactive organic compounds derived from acids via formal intermolecular dehydration; thus, acetic acid (1), on loss of water, forms acetic anhydride (2). Anhydrides of straight-chain acids containing from 2 to 12…

An explosive, poisonous, flammable liquid, boiling at 77.3°C (171°F), partly soluble in water. The formula CH2 CH C N indicates it may be regarded as vinyl cyanide, and its systematic name is 2-propenonitrile. Acrylonitrile is prepared by…

The series of elements beginning with actinium (atomic number 89) and including thorium, protactinium, uranium, and the transuranium elements through the element lawrencium (atomic number 103). These elements, chemically similar, have a strong…

A chemical element, Ac, atomic number 89, and atomic weight 227.0. Actinium was discovered by A. Debierne in 1899. Milligram quantities of the element are available by irradiation of radium in a nuclear reactor. Actinium-227 is a beta-emitting…

A powdered, granular, or pelleted form of amorphous carbon characterized by very large surface area per unit volume because of an enormous number of fine pores. Activated carbon is capable of collecting gases, liquids, or dissolved substances on the…

A technique in which a neutron, charged particle, or gamma photon is captured by a stable nuclide to produce a different, radioactive nuclide which is then measured. The technique is specific, highly sensitive, and applicable to almost every element…

The activity of a substance is a thermodynamic property that is related to the chemical potential of that substance. Activities are closely related to measures of concentration, such as partial pressures and mole fractions, and are more convenient…

A process in which a hydrogen atom in an organic compound is replaced by an acyl group (RCO where R = an organic group). The reaction involves substitution by a nucleophile (electron donor) at the electrophilic carbonyl group (CO) of a carboxylic…

A material capable of fastening together two other materials by means of surface attachment. The terms glue, mucilage, mastic, and cement are synonymous with adhesive. In a generic sense, an adhesive is any material capable of fastening by means of…

A process in which atoms or molecules move from a bulk phase (that is, solid, liquid, or gas) onto a solid or liquid surface. An example is purification by adsorption where impurities are filtered from liquids or gases by their adsorption onto the…

A suspension of small particles in a gas. The particles may be solid or liquid or a mixture of both. Aerosols are formed by the conversion of gases to particles, the disintegration of liquids or solids, or the resuspension of powdered material.…

One of a class of organic chemical compounds represented by the general formula RCHO. Aldehydes are closely related to ketones, since both types of compounds contain the carbonyl group In the case of ketones, the carbonyl group is attached to…

An organic compound that contains one or more closed rings of carbon atoms. The term alicyclic specifically excludes carbocyclic compounds with an array of π-electrons characteristic of aromatic rings. Compounds with one to five alicyclic rings of…

Any compound having highly basic properties, strong acrid taste, and ability to neutralize acids. Aqueous solutions of alkalies are high in hydroxyl ions, have a pH above 7, and turn litmus paper from red to blue. Caustic alkalies include sodium…

Usually calcium, strontium, and barium, the heaviest members of group 2 of the periodic table (excepting radium). Other members of the group are beryllium, magnesium, and radium, sometimes included among the alkaline-earth metals. Beryllium…

An organic compound with the general formula CnH2n+2. Alkanes are open-chain (aliphatic or noncyclic) hydrocarbons with no multiple bonds or functional groups. They consist of tetrahedral carbon atoms, up to 105 carbons or more in length. The C-C σ…

A hydrocarbon that contains a carbon-carbon double bond (C C); dienes or polyenes contain two or more double bonds. The synonym olefin is often used, especially in connection with industrial-scale production and applications. The term unsaturated…

A hydrocarbon that contains a triple carbon bond, C C. The first compound in the series is acetylene. Names of alkynes are formed by changing the -ane ending of the corresponding alkane to -yne and numbering as needed, for example, 2-pentyne…

A colorless to white crystalline substance which occurs naturally as the mineral kalunite and is a constituent of the mineral alunite. Alum is produced as aluminum sulfate by treating bauxite with sulfuric acid to yield alum cake or by treating the…

A chemical element, Al, with an atomic number of 13, and an atomic weight of 26.982. Pure aluminum is soft and lacks strength, but it can be alloyed with other elements to increase strength and impart a number of useful properties. Alloys of…

A chemical element, symbol Am, atomic number 95. The isotope 241Am is an alpha emitter with a half-life of 433 years. Other isotopes of americium range in mass from 232 to 247, but only the isotopes of mass 241 and 243 are important. The isotope…

A derivative of a carboxylic acid with general formula (1),where R is hydrogen or an alkyl or aryl radical. Amides are divided into subclasses, depending on the number of substituents on nitrogen. The simple, or primary, amides are considered to be…

A member of a group of organic compounds which can be considered as derived from ammonia by replacement of one or more hydrogens by organic radicals. Generally amines are bases of widely varying strengths, but a few which are acidic are known.Amines…

One of a group of complex compounds formed by the coordination of ammonia molecules with metal ions and, in a few instances, such as calcium, strontium, and barium, with metal atoms. Some typical examples of ammines include [Co(NH3)6]Cl2 (rose),…

A product of a reaction between ammonia, NH3, and various acids. The general reaction for formation is (1).1 Examples of ammonium salts are ammonium chloride, NH4Cl, ammonium nitrate, NH4NO3, and ammonium carbonate, (NH4)2CO3. These compounds are…

The element is dimorphic, existing as a yellow, metastable form composed of Sb4 molecules, as in antimony vapor and the structural unit in yellow antimony; and a gray, metallic form, which crystallizes with a layered rhombohedral structure. Antimony…

A substance that, when present at a lower concentration than that of the oxidizable substrate, significantly inhibits or delays oxidative processes, while being itself oxidized. For antioxidants, such as ascorbic acid (Fig. 1), antioxidative…

A mixture of one part by volume of concentrated nitric acid and three parts of concentrated hydrochloric acid. Aqua regia was so named by the alchemists because of its ability to dissolve platinum and gold. Either acid alone will not dissolve these…

A hydrocarbon with a chemistry similar to that of benzene. Aromatic hydrocarbons are either benzenoid or nonbenzenoid. Benzenoid aromatic hydrocarbons contain one or more benzene rings and are by far the more common and the more important…

The application of chemistry to the technical examination, authentication, and preservation of cultural property. Chemists working in museums engage in a broad range of investigations, most frequently studying the chemical composition and structure…

A chemical element, At, atomic number 85. Astatine is the heaviest of the halogen groups, filling the place immediately below iodine in group 17 of the periodic table. Astatine is a highly unstable element existing only in short-lived radioactive…

A chemical reaction or series of reactions leading to the predominant or exclusive formation of a single enantiomer, that is, one of an isomeric pair of chemical compounds whose molecules are nonsuperimposable mirror images. Enantiomers are said to…

The maximum incorporation of the starting materials into the final product for any chemical reaction.A broad imperative for synthetic chemists today is the need to develop innovative and sophisticated synthetic methods to keep pace with the…

The mass of an atom or molecule on a scale where the mass of a carbon-12 (12C) atom is exactly 12.0 atomic mass units. The mass of any atom is approximately equal to the total number of its protons and neutrons multiplied by the atomic mass unit, u…

The number of elementary positive charges (protons) contained within the nucleus of an atom. It is denoted by the letter Z. Correspondingly, it is also the number of planetary electrons in the neutral atom. Relation to x-ray wavelengths The concept…

An alkaloid, C17H23NO3, with the chemical structure below. The systematic chemical name is endo(±)- α-(hydroxymethyl)phenylacetic acid 8-methyl-8-azabicyclo[3.2.1]oct-3-yl ester, and in phamacy it is sometimes known as dl-hyoscyamine. It occurs in…

Auger electron spectroscopy (AES) is a widely used technique that detects the elements in the first atomic layers of a solid surface. Although many elements can be detected, hydrogen usually cannot be observed. Excellent spatial resolution can be…

The slow, flameless combustion of materials by reaction with oxygen; sometimes spelled autooxidation. Autoxidation is important because it is a useful reaction for converting compounds to oxygenated derivatives, and also because it occurs in…

The principle that equal volumes of all gases and vapors, under the same conditions of temperature and pressure, contain an identical number of molecules; also known as Avogadro's hypothesis. From Avogadro's law the converse follows that equal…

The number of elementary entities in one mole of a substance. A mole is defined as an amount of a substance that contains as many elementary entities as there are atoms in exactly 12 g of 12C; the elementary entities must be specified and may be…

A solution of two or more liquids, the composition of which does not change upon distillation. The composition of the liquid phase at the boiling point is identical to that of the vapor in equilibrium with it, and such mixtures or azeotropes form…

A compound containing the group N3, which can be represented as a resonance hybrid of two structures, as shown in expression (1). 1 Sodium azide, from which most other azides are prepared, is manufactured by passing nitrous oxide over heated…

An instrument for weighing. The word “balance,” derived from the Latin bilanx (“having two pans”), is often used interchangeably with “scale” or “scales” (Old English, meaning dishes or plates). However, balance is the preferred term for an…

A chemical element, Ba, with atomic number 56 and atomic weight of 137.34. Barium is eighteenth in abundance in the Earth's crust, where it is found to the extent of 0.04%, making it intermediate in amount between calcium and strontium, the other…

In the Brönsted-Lowry classification, any chemical species, ionic or molecular, capable of accepting or receiving a proton (hydrogen ion) from another substance. The other substance acts as an acid in giving up the proton. A substance may act as a…

An electrochemical device that stores chemical energy which can be converted into electrical energy, thereby providing a direct-current voltage source. Although the term “battery” is properly applied to a group of two or more electrochemical cells…

A colorless liquid aromatic hydro­carbon, formula C6H6, also know as benzol, which boils at 80.1°C (176°F) and freezes at 5.5°C (41.9°F).Benzene ( 1 ) is an important chemical intermediate used to produce ethylbenzene (used to make styrene),cumene…

A chemical compound, sometimes called dibenzoyl peroxide, with the formula shown below. It is a colorless, crystalline solid, melting point 106–108°C (223–226°F), that is virtually insoluble in water but very soluble in most organic solvents.…

Element number 97, symbol Bk, the eighth member of the actinide series of elements. In this series the 5f electron shell is being filled, just as the 4f shell is being filled in the lanthanide (rare-earth) elements. These two series of elements are…

A chemical element, Be, atomic number 4, with an atomic weight of 9.0122. Beryllium, a rare metal, is one of the lightest structural metals, having a density about one-third that of aluminum. Some of the important physical and chemical properties of…

A method for quantitatively determining the concentration, biological activity, or potency of a substance by its effect on the growth of a suitable animal, plant, or microorganism under controlled conditions. A bioassay (biological assay) is a…

A subclass of fuel cells in which the catalyst at the anode and/or the cathode are of biological origin (that is, nucleic acid, enzyme, microbe, or organelle). When considering energy storage, the most common form of energy storage today is the…

The field at the interface between biochemistry and inorganic chemistry; also known as inorganic biochemistry or metallobiochemistry. This field involves the application of the principles of inorganic chemistry to problems of biology and…

A biochemical reaction involving the transfer of a negatively charged electron from one organic compound to another organic compound or to oxygen. Biological oxidation is an energy-producing reaction in living cells, and it is coupled with a…

The science that describes the structure, reactivity, and interactions of organic compounds of biological significance. It represents the meeting of biochemistry, in its broadest sense, with organic chemistry in an attempt to describe and control…

A macromolecule derived from natural sources; also known as biological polymer. Nature has developed effective means for efficiently preparing and utilizing biopolymers. Some are used as structural materials, food sources, or catalysts. Others have…

An integrated device consisting of a biological recognition element and a transducer capable of detecting the biological reaction and converting it into a signal that can be processed. Ideally, the sensor should be self-contained, so that it is not…

The synthesis of more complex molecules from simpler ones in cells by utilizing biosynthetic pathways consisting of a series of reactions mediated by enzymes. The overall economy and survival of the cell is governed by the interplay between the…

The metallic element, Bi, of atomic number 83 and atomic weight 208.980 belonging in the periodic table to group 15. Bismuth is the most metallic element in this group in both physical and chemical properties. The only stable isotope is that of mass…

A chemical element, symbol Bh, atomic number 107. Bohrium was synthesized and identified in 1981 by using the Universal Linear Accelerator (UNILAC) of the Gesellschaft für Schwerionenforschung (GSI) at Darmstadt, West Germany, by a team led by P.…

The angle between two bonds sharing a common atom is known as the bond angle. The distance between the nuclei of bonded atoms is known as bond distance. The geometry of a molecule can be characterized by bond angles and distances. A chain of four…

The general name given to any one of the binary boron hydrides of the general formula BnHm. Boron is the only element other than carbon that exhibits an extensive series of molecular hydrides. In contrast to the chains and rings of carbon atoms that…

A chemical element, B, atomic number 5, atomic weight 10.811, in group 13 of the periodic table. It has three valence electrons and is nonmetallic in behavior. It is classified as a metalloid and is the only nonmetallic element which has fewer than…

A law of gases which states that at constant temperature the volume of a gas varies inversely with its pressure. This law, formulated by Robert Boyle (1627–1691), can also be stated thus: The product of the volume of a gas times the pressure exerted…

A polymer chain having branch points that connect three or more chain segments. Examples of branched polymers include long chains having occasional and usually short branches comprising the same repeat units as the main chain (nominally termed a…

A chemical element, Br, atomic number 35, atomic weight 79.909, which normally exists as Br2, a dark-red, low-boiling but high-density liquid of intensely irritating odor. This is the only nonmetallic element that is liquid at normal temperature and…