Science, Tech, Math › Science Predicting Formulas of Ionic Compounds A Worked Example Problem Share Flipboard Email Print Markus Brunner, Creative Commons License 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 July 12, 2019 This problem demonstrates how to predict the molecular formulas of ionic compounds. Problem Predict the formulas of the ionic compounds formed by the following elements: lithium and oxygen (Li and O)nickel and sulfur (Ni and S)bismuth and fluorine (Bi and F)magnesium and chlorine (Mg and Cl) Solution First, look at the locations of the elements on the periodic table. Atoms in the same column as each other (group) tend to exhibit similar characteristics, including the number of electrons the elements would need to gain or lose to resemble the nearest noble gas atom. To determine common ionic compounds formed by elements, keep the following in mind: Group I ions (alkali metals) have +1 charges.Group 2 ions (alkaline earth metals) have +2 charges.Group 6 ions (nonmetals) have -2 charges.Group 7 ions (halides) have -1 charges.There is no simple way to predict the charges of the transition metals. Look at a table listing charges (valences) for possible values. For introductory and general chemistry courses, the +1, +2, and +3 charges are most often used. When you write the formula for an ionic compound, remember that the positive ion is always listed first. Write down the information you have for the usual charges of the atoms and balance them to answer the problem. Lithium has a +1 charge and oxygen has a -2 charge, therefore2 Li+ ions are required to balance 1 O2- ion.Nickel has a charge of +2 and sulfur has a -2 charge, therefore1 Ni 2+ ion is required to balance 1 S2- ion.Bismuth has a +3 charge and Fluorine has a -1 charge, therefore1 Bi3+ ion is required to balance 3 F- ions.Magnesium has a +2 charge and chlorine has a -1 charge, therefore1 Mg2+ ion is required to balance 2 Cl- ions. Answer Li2ONiSBiF3MgCl2 The charges listed above for atoms within groups are the common charges, but you should be aware that the elements sometimes take on different charges. See the table of the valences of the elements for a list of the charges that the elements have been known to assume.