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Chemistry: Chemistry Glossary

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Chemistry Glossary

CHEMISTRY FUNDAMENTALS: A Glossary   Chemistry Resources Locator
Prepared by April M. Love, University of California, Irvine (2003)


  1. the science of atoms (elements), their structure and behavior;
  2. the composition and properties of compounds;
  3. the reactions between substances and their subsequent energy exchange;
  4. the laws that unite these phenomena into a comprehensive system.












ELEMENTS are the most basic form of chemical identity. The smallest unit of any element is the atom. The atomic element can only be broken down further into electrons, protons, and neutrons. There are now 109 kinds of substances that make up all matter at and above the atomic level.

MOLECULES are chemical units composed of one or more atoms help together by chemical bonds. The simplest molecules have only one kind of atom, e.g., helium molecules, which have one atom per molecule. The bonds hold the elemental atoms in certain spatially arranged shapes, called "structures," which depend on: (1) what elements are involved in the molecule and (2) in the placement of each atom within the molecule.

COMPOUNDS are substances made up of atoms of two of more elements united by chemical bonds or valence forces. A compound is homogeneous; the elements have definite proportions by weight and are represented by a molecular formula. To take a compound apart, a chemical reaction must happen, energy (as heat) or an electrical current must be applied from an external source.

MIXTURES are made up of different molecules that are blended together. No chemical reactions take place, and no chemical bonds are formed between molecules. The molecules in the mixture can be separated mechanically. Gasoline is an example of a liquid mixture. Air is a natural mixture in the gaseous state.

PERIODIC TABLE OF THE ELEMENTS is an arrangement of the chemical elements by symbol in a geometric pattern designed to represent the relationship of atomic number and atomic weight. The elements are arranged in increasing order of the atomic number and atomic weight and are placed in groups within "periods" or families, which have similar functional chemical activities, beginning with the alkali metals and ending with an element of the helium family. The chief function of the periodic table is to serve as a fundamental framework for the systematic organization of chemistry. The CRC Handbook has an example of the Periodic Table of the Elements inside the cover.

INORGANIC CHEMISTRY is a major branch of chemistry, which focuses on all compounds except "hydrocarbons" (compounds made up of carbon and hydrogen). Carbon-oxygen compounds (e.g. carbon monoxide and carbon dioxide) and carbon-sulfur compounds (e.g. carbon disulfide) are inorganic compounds. The inorganic section of the CRC Handbook of Chemistry and Physics is arranged alphabetically by the letter symbol for the element, e.g. "O" for oxygen, "W" for tungsten ("wolfram" is the German word for that element, hence the "W".), "Rn" for radon, "Ag" for silver and "Au" for gold.

ORGANIC CHEMISTRY is a major branch of chemistry, which focuses on molecular combinations of carbon and hydrogen. There may be other elements present, but the overall presence of carbon and hydrogen has been defined as "organic chemistry. The naming conventions in organic chemistry are one of the major obstacles in finding information about organic compounds. The International Union of Pure and Applied chemists (IUPAC) is the deciding group for the current names of organic compounds. Older names, for example "naphtha," "toluene," "phenol," and the like do not follow the IUPAC naming rules. Yet, these older names continue to be used in the literature and by working chemists.

MOLECULAR FORMULA is one way to organize chemical information. The chemical formulas are organized in indexes by: (1) the number of carbon atoms, (2) the number of hydrogen atoms, and then (3) alphabetically by chemical symbol of the element, e.g. "Al" aluminum, "F" fluorine, etc in order of atoms for each element. For example, "C6H12O6" is the molecular formula for sugar.

ORGANOMETALLIC COMPOUNDS are made up of a metal atom or molecule attached directly to carbon atoms. Inorganic chemists' work often overlaps that of organic chemists in their interest in organometallic compounds.

HALOGEN is one of the electronegative elements of Group VIIA of the Periodic Table of the Elements. Halogens include the elements fluorine, chlorine, bromine, iodine and astatine. Fluorine is the most reactive of these elements.

HETEROCYCLIC COMPOUNDS are compounds with a closed-ring structure with one or more atoms in the ring which are elements other than carbon, e.g. nitrogen, sulfur, oxygen, etc.