Glossary of chemical terms

absorbance
	Absorbance (A) is defined as -log(1-α) = log(1/τ), where α is the absorptance and τ the transmittance of a medium through which a light beam passes.
absorbed dose
	For any ionising radiation, absorbed dose (D) is the mean energy imparted to an element of irradiated matter divided by the mass of that element.
absorptance
	Absorptance (α) is ratio of the radiant or luminous flux in a given spectral interval absorbed in a medium to that of the incident radiation. Also called absorption factor.
absorption coefficient
	Absorption coefficient (a) is the relative decrease in the intensity of a collimated beam of electromagnetic radiation, as a result of absorption by a medium, during traversal of an infinitesimal layer of the medium, divided by the length traversed.
absorption coefficient, molar
	Molar absorption coefficient (ε) is absorption coefficient divided by amount-of-substance concentration of the absorbing material in the sample solution (ε = a/c). The SI unit is m2mol-1. Also called extinction coefficient, but usually in units of dm3cm-1mol-1.
acid
	Acid is a type of compound that contains hydrogen and dissociates in water to produce positive hydrogen ions. The reaction, for an acid HA is commonly written: HA H+ + A- In fact, the hydrogen ion (the proton) is solvated, and the complete reaction is: HA + H2O H3O+ + A- This definition of acids comes from the Arrhenius theory. Such acids tend to be corrosive substances with a sharp taste, which turn litmus red and give colour changes with other indicators. They are referred to as protonic acids and are classified into strong acids, which are almost completely dissociated in water (e.g. sulphuric acid and hydrochloric acid), and weak acids, which are only partially dissociated (e.g. acetic acid and hydrogen sulphide). The strength of an acid depends on the extent to to which it dissociates, and is measured by its dissociation constant. In the Lowry-Brønsted theory of acids and bases (1923), the definition was extended to one in which an acid is a proton donor (a Brønsted acid), and a base is a proton acceptor (a Brønsted base). An important feature of the Lowry-Brønsted concept is that when an acid gives up a proton, a conjugate base is formed that is capable of accepting a proton. acid base + H+ Similarly, every base produces its conjugate acid as a result of accepting a proton. base + H+ acid For example, acetate ion is the conjugate base of acetic acid, and ammonium ion is the conjugate acid of ammonia. As the acid of a conjugate acid/base pair becomes weaker, its conjugate base becomes stronger and vice versa. A further extension of the idea of acids and bases was made in the Lewis theory. In this, a G. N. Lewis acid is a compound or atom that can accept a pair of electrons and a Lewis base is one that can donate an electron pair. This definition encompasses "traditional" acid-base reactions, but it also includes reactions that do not involve ions, e.g. H3N: BCl3 → H3NBCl3 in which NH3 is the base (donor) and BCl3 the acid (acceptor).
acid dissociation constant
	Acid dissociation constant (Ka) is the equilibrium constant for the dissociation of an acid HA through the reaction HA + H2O A- + H3O+ The quantity pKa = -log Ka is often used to express the acid dissociation constant. pKa = -log Ka
actinides
	The actinides (actinons or actinoids) are the fourteen elements from thorium to lawrencium inclusive, which follow actinium in the periodic table. The position of actinium is somewhat equivocal and, although not itself an actinide, it is often included with them for comparative purpose. The series includes the following elements: thorium (Th), protactinium (Pa), uranium (U), neptunium (Np), plutonium (Pu), amercium (Am), curium (Cm), berkelium (Bk), californium (Cf), einsteinium (Es), fermium (Fm), mendelevium (Md), nobelium (No) and lawrencium (Lr). Every known isotope of the actinide elements is radioactive. Traces of Pa, Np and Pu are consequently found, but only Th and U occur naturally to any useful extent.
activation energy
	In general, activation energy is the energy that must be added to a system in order for a process to occur, even though the process may already be thermodynamically possible. In chemical kinetics, the activation energy is the height of the potential barrier separating the products and reactants. It determines the temperature dependence of the reaction rate.
activity
	Activity (a) is a thermodynamic function used in place of concentration in equilibrium constants for reactions involving nonideal gases and solutions. For the species i activity is defined as a = f × c where ai is the activity of the species i, ci is its molar concentration, and fi is a dimensionless quantity called the activity coefficient.
activity coefficient
	Activity coefficient (γ or f) is a fractional number which when multiplied by the molar concentration of a substance in solution yields the chemical activity. This term gives an idea of how much interaction exists between molecules at higher concentration. In solutions of very low ionic strength, when m is less than 0.01, the Debye-Hückel limiting law can be used to calculate approximate activity coefficients where γi = activity coefficient of the species i, zi = charge on the species i and μ = ionic strength of the solution.
adiabatic process
	Adiabatic process is a thermodynamic process in which no heat enters or leaves the system. In general, an adiabatic change involves a fall or rise in temperature of the system.
adsorption
	Adsorption is a process in which molecules of gas, of dissolved substances in liquids, or of liquids adhere in an extremely thin layer to surfaces of solid bodies with which they are in contact.
aerosol
	Aerosols are colloidal dispersions of liquid or solid particles in a gas, as in a mist or smoke. The commonly used aerosol sprays contain an inert propellant liquefied under pressure. The pressure of the gas causes the mixture to be released as a fine spray (aerosol) or foam (aerogel) when a valve is opened.
alcohols
	Alcohols are compounds in which a hydroxy group, -OH, is attached to a saturated carbon atom.
aldehydes
	Aldehydes are broad class of organic compounds having the generic formula RCHO, and characterized by unsaturated carbonyl group (C=O). They are formed from alcohols by either dehydrogenation or oxidation. Their chemical derivation is indicated by the name al(cohol) + dehyd(rogenation).
aliphatic compounds
	Aliphatic compounds are acyclic or cyclic, saturated or unsaturated carbon compounds, excluding aromatic compounds.
alkali metals
	Alkali metal is a term that refers to six elements: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). These elements make up group 1 of the periodic table of elements. They all form singly charged positive ions, and are extremely reactive. They react violently with water, forming hydroxides and releasing hydrogen gas and heat. Caesium and francium are the most reactive and lithium is the least.
alkaline earth metals
	Alkali earth metal is a term that refers to six elements: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These elements make up group 2 of the periodic table of elements. They all exhibit a single oxidation state, +2. They are all light and very reactive. Barium and radium are the most reactive and beryllium is the least. Chemists to denote slightly soluble metal oxides formerly used the term "earth". The oxides of barium, strontium, and calcium resemble alumina (Al2O3), a typical "earth", but form alkaline mixtures with water. For this reason barium, strontium, and calcium were called alkaline earth metals. This name has now been extended to include all of the elements of group 2.
alkaloids
	Alkaloids are basic nitrogen organic compounds (mostly heterocyclic) derived from plants and having diverse pharmacological properties. Alkaloids include morphine, cocaine, atropine, quinine, and caffeine, most of which are used in medicine as analgesics or anaesthetics. Some alkaloids are poisonous, e.g. strychnine and coniine, and colchicine inhibit cell division.
alkanes
	Alkanes (paraffins) are acyclic branched or unbranched hydrocarbons having the general formula CnH2n+2, and therefore consisting entirely of hydrogen atoms and saturated carbon atoms. In systematic chemical nomenclature alkane names end in the suffix -ane. They form a homologous series (the alkane series) methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), etc. The lower members of the series are gases; the high-molecular mass alkanes are waxy solid. Generaly the alkanes are fairly unreactive. They form haloalkanes with halogens when irradiated with ultraviolet radiation. Alkanes are present in natural gas and petroleum.
alkenes
	Alkenes are acyclic branched or unbranched hydrocarbons having one or more double carbon-carbon bonds in their molecules. In systematic chemical nomenclature, alkene names end in the suffix -ene. The general formula is CnH(2n+2)-2x were x is the number of double bonds. Alkenes that have only one double bond form a homologous series: ethene (ethylene), CH2=CH2, propene, CH3CH2=CH2, etc. Alkenes typically undergo addition reactions to the double bond.
alkynes
	Alkynes (acetylenes) are acyclic branched or unbranched hydrocarbons having a one or more triple carbon-carbon bond. In systematic chemical nomenclature alkine names end in the suffix -yne. The general formula is CnH(2n+2)-4x were x is the number of triple bonds. Alkynes that have only one triple bond form a homologous series: ethyne (acetylene), CH≡CH, propyne, CH3CH≡CH, etc. Like alkenes, alkynes undergo addition reaction.
allotropy
	Allotropy is the occurrence of an element in two or more crystalline forms.
alpha particle
	Alpha particle is a helium nucleus emitted spontaneously from radioactive elements both natural and manufactured. Its energy is in range 4-8 MeV and is dissipated in a very short path, i.e. a few centimetres of air or less than 0.005 mm of aluminium. As a helium nucleus consists of two protons and two neutrons bound together as a stable entity the loss of an alpha particle involves a decrease in nucleon number of 4 and decrease of 2 in the atomic number, e.g. A stream of alpha particles is known as an alpha ray or alpha-radiation.
amines
	Amines are compounds formally derived from ammonia by replacing one, two, or three hydrogen atoms by hydrocarbyl groups, and having the general structures RNH2 (primary amines), R2NH (secondary amines), R3N (tertiary amines).
amino acids
	Amino acids are compounds containing both a carboxylic acid group (-COOH) and an amino group (-NH2 ). The most important are the a-amino acids, in which the -NH2 group in attached to the C atom adjacent to the -COOH group. In the β-amino acids, there is an intervening carbon atom.
ampere
	Ampere (A) is the SI base unit of electric current. The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10-7 newton per metre of length.
angstrom
	Ångström (Å) is a unit of length equal to 10-10 m. The Ångström is defined in terms of the wavelength of the emission spectra's red line of an atom of cadmium (6438.4696 Å).
anion
	Anion is a negatively charged atomic or molecular particle.
anion exchange
	An anionic resin has negative ions built into its structure and therefore exchanges positive ions. In anion exchange, the side groups are ionized basic groups, such as (-NH2, -NRH, -NR2, -NR3+) to which anions OH- are attached. The exchange reaction is one in which different anions in the solution displace the OH- from the solid.
anisotropy
	Anisotropy is the property of molecules and materials to exhibit variations in physical properties along different molecular axes of the substance.
antiparticle
	Antiparticle is a particle having the same mass as a given elementary particle and a charge equal in magnitude but opposite in sign.
ar
	Ar is symbol for element argon.
are
	Are (a) is a unit of area equal to 100 m2. The unit is still used in agriculture.
arenes
	Arenes are monocyclic and polycyclic aromatic hydrocarbons. See aromatic compounds.
aromatic compounds
	Aromatic compounds are major group of unsaturated cyclic hydrocarbons containing one or more rings, typified by benzene, which has a 6-carbon ring containing three double bonds. All the bonds in benzene (C6H6) are the same length intermediate between double and single C-C bonds. The properties arise because the electrons in the p-orbitals are delocalised over the ring, giving extra stabilization energy of 150 kJ/mol over the energy of Kekulé structure. Aromatic compounds are unsaturated compounds, yet they do not easily partake in addition reactions. Historical use of the term implies a ring containing only carbon (e.g., benzene, naphthalene), but it is often generalized to include heterocyclic structures such as pyridine and thiophene.
astronomical unit
	Astronomical unit (AU) is a unit of length employed in astronomy for describing planetary distance. It is the mean distance of the earth from the sun, equal to 1.49597870×1011 m.
atomic number
	Atomic number (Z) is a characteristic property of an element, equal to the number of protons in the nucleus.
atomic weight
	See Relative atomic mass
autoignition temperature
	Autoignition temperature is the minimum temperature required to initiate or cause self-sustained combustion in any substance in the absence of a spark or flame. This varies with the test method.
Avogadro constant
	Avogadro constant (NA or L) is the number of elementary entities in one mole of a substance. L = N / n It has the value (6.022 045 ± 0.000 031) × 1023 mol-1.
azeotrope
	Azeotrope is a mixture of two liquids that boils at constant composition, i.e. the composition of the vapour is the same as that of the liquid. Azeotropes occur because of deviations in Raoult's law leading to a maximum or minimum in the boiling point - composition diagram. The composition of an azeotrope depends on the pressure.
azo compounds
	Azo compounds are organic compounds containing the group -N=N- linking two other groups. They can be formed by reaction of a diazonium ion with a benzene ring.

 

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