Compounds that contain both metals and nonmetals are usually ionic. For example, Na2SO4 contains a metal (Na) and nonmetals (sulfur and oxygen), and so is expected to be ionic. But CO2 and CH4 contain only nonmetals, and are expected to be molecular compounds.
The rule works because metals give up electrons very easily to form positive ions (cations), and nonmetals gain electrons easily to form negative ions (anions). Electron transfer usually occurs when a metal and a nonmetal react, forming an ionic compound.
You have to watch out for a few exceptions.
Author: Fred Senese firstname.lastname@example.org
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/compounds/faq/print-distinguishing-molecular-and-ionic.shtml