addition compound

An addition compound contains two or more simpler compounds that can be packed in a definite ratio into a crystal. A dot is used to separate the compounds in the formula. For example, ZnSO₄·7 H₂O is an addition compound of zinc sulfate and water. This represents a compound, and not a mixture, because there is a definite 1:7 ratio of zinc sulfate to water in the compound. Hydrates are a common type of addition compound.

adiabatic ionization energy. Compare with vertical ionization energy.

The lowest energy required to remove an electron from an atom, ion, or molecule in the gas phase. The adiabatic ionization energy is the difference between the ground state energy of the ion formed and the energy of the original atom, molecule, or ion.

activated complex. transition state.

An intermediate structure formed in the conversion of reactants to products. The activated complex is the structure at the maximum energy point along the reaction path; the activation energy is the difference between the energies of the activated complex and the reactants.

accelerator.

1. A substance that makes vulcanization of rubber occur more quickly or at a lower temperature. 2. A substance that makes crosslinking in a polymer occur more quickly or at a lower temperature, e. g., accelerators are added to Super Glue to make it set up quickly.

absorption

1. Penetration of molecules into the bulk of a solid or liquid, forming either a solution or compound. Absorption can be a chemical process (a strong solution of NaOH absorbs CO₂ from the air) or a physical process (palladium absorbs hydrogen gas). 2. Capture and transformation of energy by a substance; for example, copper looks reddish because it absorbs blue light. An absorbent captures another material and distributes it throughout; an adsorbent captures another material and distributes it on its surface only.

absolute zero. (0 K)

The temperature at which the volume of an ideal gas becomes zero; a theoretical coldest temperature that can be approached but never reached. Absolute zero is zero on the Kelvin scale, -273.15°C on the Celsius scale, and -459.67°F on the Fahrenheit scale.