Science, Tech, Math › Science Monatomic Ion Definition and Examples Learn What a Monatomic Ion Is in Chemistry Share Flipboard Email Print Sodium chloride dissociates into monatomic sodium and chloride ions in water. LAGUNA DESIGN / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated June 04, 2019 A monatomic ion is an ion formed from a single atom. In other words, it is a single atom that has a different number of protons and electrons. The charge on the ion is the difference between the number of protons and electrons. If there are more protons, the charge is positive. If there is an excess of electrons, the charge is negative. Metals typically form cations, while nonmetals usually form anions. Examples KCl dissociates in water into K+ and Cl- ions. Both of these ions are monatomic ions. Ionization of an oxygen atom may result in O2-, which is a monatomic ion. Hydrogen usually forms the monatomic ion H+, however, it sometimes acts as an anion and forms H-. Monatomic Ion Versus Monatomic Atom Technically, a monatomic ion is a form of monatomic atom. However, the term "monatomic atom" usually refers to neutral atoms of elements. Examples include atoms of krypton (Kr) and neon (Ne). While krypton, neon, and other noble gases typically exist as monatomic atoms, they rarely form ions. Source William Masterton; Cecile Hurley (2008). Chemistry: Principles and Reactions. Cengage Learning. p. 176. ISBN 0-495-12671-3.