Air & Water Quality Glossary Valence Valence is a number that is assigned to an ion or atom that indicates the ion's or atom's ability to combine with other atoms or ions. The valence is often shown with either a roman numeral after the symbol for the element or with a superscript number with a +/- symbol. Example: iron may be written as Fe (III) or Fe+3 . This is pronounced as "iron three" Previous Page Home | Top of page | Contact © Copyright 2000, Air & Water Quality, Inc. Maine's Water Experts