mon·a·tom·ic (m?n??-t?m??k)adj. Occurring as single atoms: Helium is a monatomic gas.mon?a·tom?ic·al·ly adv.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.monatomic (?m?n??t?m?k) or monoatomicadj1. (Chemistry) (of an element) having or consisting of single atoms: argon is a monatomic gas. 2. (Chemistry) (of a compound or molecule) having only one atom or group that can be replaced in a chemical reaction3. (Chemistry) a less common word for monovalentCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014mon?a?tom?ic (?m?n ??t?m ?k) adj. 1. having one atom in the molecule. 2. containing one replaceable atom or group. 3. having a valence of one. [1840?50] mon`a?tom?i?cal?ly, adv. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.