Category: Chemistry Dictionary

Atomic mass unit : See unified atomic mass unit.

Atomic mass : The mass of an atom, typically expressed in unified atomic mass units and nearly equivalent to the mass number.

Atom : A chemical element in its smallest form, made up of protons and neutrons within the nucleus and electrons circling the nucleus.

Aryl : Any functional group or substituent derived from an aromatic ring, such as phenyl or naphthyl. The symbol Ar is often used as a placeholder for a generic aryl group in structural diagrams.

Aqueous solution : A solution in which the solvent is water. It is denoted in chemical equations by appending (aq) to a chemical formula.

Aromaticity : A chemical property of conjugated rings of atoms, such as benzene, which results in unusually high stability. Such rings are said to be aromatic.

Anion : A negatively charged ion. I.e. an atom that has an excess of electrons compared to protons.

Anode : 1.  An electrode through which the conventional electric current (the flow of positive charges) enters into a polarized electrical circuit.2.  The wire or plate of an electrochemical cell having an excess positive charge. Negatively charged anions always move toward the anode. Contrast cathode.

Analytical chemistry : The branch of chemistry which studies and makes use of instruments and methods to separate, quantify, and identify chemical substances, both by classical wet chemistry techniques such as precipitation, extraction, distillation, and observational analysis, and by modern instrumental techniques such as chromatography, spectroscopy, and electrochemistry.

Amount of substance: Also enplethy, chemical amount, or simply amount.

The number of discrete particles (such as molecules, atoms, ions, electrons, or any other atomic-scale entity) in a given sample of matter, divided by the Avogadro constant. The SI base unit for amount of substance is the mole (mol).

Amalgam : Any alloy of mercury with another metal.

Alloy : A mixture of metals or of a metal and another element which in combination exhibit a metallic bonding character. Common examples include bronze, brass, and pewter.

Allotrope : Elements that can have different structures (and therefore different forms), such as carbon (diamonds, graphite, and fullerene).

Allomer : A substance that differs in chemical composition but has the same crystalline structure as another substance.

Alkyne : An unsaturated hydrocarbon containing at least one pair of triple-bonded carbons.

Alkyl group : A functional group consisting of an alkane missing a hydrogen atom.

Alkaline earth metal : Any of the metallic elements belonging to Group 2 of the periodic table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

Alkane : Any saturated acyclic hydrocarbon.

Aldehyde : Any organic compound consisting of a carbonyl group attached to a hydrogen atom and any other R-group.

Alkali metal : Any of the metallic elements belonging to Group 1 of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr).

Aeration : The mixing of air into a liquid or a solid.

Alcohol : Any organic compound consisting of a hydroxyl functional group attached to a saturated carbon atom.

Adsorption : The chemical adhesion of atoms, ions, or molecules of one substance (either a gas, liquid, or dissolved solid) to the surface of another substance, resulting in a film of the first substance being weakly bonded to the interface between the two substances. Adsorption differs from absorption in that it is exclusively a surface phenomenon, while absorption involves entire volumes of materials.

Adhesion : The tendency of dissimilar particles or surfaces to cling to one another as a result of intermolecular forces. Contrast cohesion.

Addition reaction : In organic chemistry, when two or more molecules combine to make a larger one.

Acyclic : Containing only linear structures of atoms (particularly in hydrocarbons).

Activation energy : The minimum energy which must be available to a chemical system with potential reactants in order to result in a chemical reaction.

Activity series : See reactivity series.

Activated complex : A structure that forms because of a collision between molecules while new bonds are formed.

Actinides: Also actinoids.

The periodic series of metallic elements with atomic numbers 89 to 103, from actinium through lawrencium.

Acid anhydride : A compound with two acyl groups bound to a single oxygen atom.

Acid dissociation constant (K_a) : Also acid ionization constant or acidity constant.

A quantitative measure of the strength of an acid in solution expressed as an equilibrium constant for a chemical dissociation reaction in the context of acid-base reactions. It is often given as its base-10 cologarithm, pK_a.

Accuracy : How close a measured value is to the actual or true value. Compare precision.

Acid : A compound which, when dissolved in water, gives a pH of less than 7.0, or donates a hydrogen ion.

Absorption: 1.  The physical or chemical process by which a substance in one state becomes incorporated into and retained by another substance of a different state. Absorption differs from adsorption in that the first substance permeates the entire bulk of the second substance, rather than just adhering to the surface.2.  The process by which matter (typically electrons bound in atoms) takes up the energy of electromagnetic radiation and transforms it into any of various types of internal energy, such as thermal energy. This type of absorption is the principle on which spectrophotometry is based.

Absolute zero : A theoretical condition concerning a system at the lowest limit of the thermodynamic temperature scale, or zero kelvins, at which the system does not emit or absorb energy (i.e. all atoms are at rest). By extrapolating the ideal gas law, the internationally agreed-upon value for absolute zero has been determined as −273.15 °C (−459.67 °F; 0.00 K).