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transition element, metal
Chem any element belonging to one of three series of elements with atomic numbers between 21 and 30, 39 and 48, and 57 and 80. They have an incomplete penultimate electron shell and tend to exhibit more than one valency and to form complexes
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
transition element[tran′zish·ən ‚el·ə·mənt]
One of a group of metallic elements in which the members have the filling of the outermost shell to 8 electrons interrupted to bring the penultimate shell from 8 to 18 or 32 electrons; includes elements 21 through 29 (scandium through copper), 39 through 47 (yttrium through silver), 57 through 79 (lanthanum through gold), and all known elements from 89 (actinium) on. Also known as transition metal.
An element used to couple one type of transmission system to another, as for coupling a coaxial line to a waveguide.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.