Science, Tech, Math › Science The Molecular Formula for Water It shows 1 oxygen atom and 2 hydrogen atoms Share Flipboard Email Print Laguna Design / Getty Images Science Chemistry Basics Chemical Laws 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 December 03, 2018 The molecular formula for water is H2O. One molecule of water consists of one oxygen atom covalently bonded to two hydrogen atoms. There are three isotopes of hydrogen. The usual chemical formula for water assumes the hydrogen atoms consist of the isotope protium (one proton, no neutrons). Heavy water is also possible, in which one or more of the atoms of hydrogen consist of deuterium (symbol D) or tritium (symbol T). Other forms of the chemical formula for water include D2O, DHO, T2O, and THO. It's theoretically possible to form TDO, although such a molecule would be extremely rare. Although most people assume water is H2O, only completely pure water lacks other elements and ions. Drinking water usually contains chlorine, silicates, magnesium, calcium, aluminum, sodium, and trace amounts of other ions and molecules. Also, water dissolves itself, forming its ions, H+ and OH-. A sample of water contains the intact water molecule along with hydrogen cations and hydroxide anions.