Predicting Formulas of Ionic Compounds

A Worked Example Problem

The alkaline earth metals have a +2 charge.
Markus Brunner, Creative Commons License

This problem demonstrates how to predict the molecular formulas of ionic compounds.


Predict the formulas of the ionic compounds formed by the following elements:

  1. lithium and oxygen (Li and O)
  2. nickel and sulfur (Ni and S)
  3. bismuth and fluorine (Bi and F)
  4. magnesium and chlorine (Mg and Cl)


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.

  1. Lithium has a +1 charge and oxygen has a -2 charge, therefore
    2 Li+ ions are required to balance 1 O2- ion.
  2. Nickel has a charge of +2 and sulfur has a -2 charge, therefore
    1 Ni 2+ ion is required to balance 1 S2- ion.
  3. Bismuth has a +3 charge and Fluorine has a -1 charge, therefore
    1 Bi3+ ion is required to balance 3 F- ions.
  4. Magnesium has a +2 charge and chlorine has a -1 charge, therefore
    1 Mg2+ ion is required to balance 2 Cl- ions.


  1. Li2O
  2. NiS
  3. BiF3
  4. MgCl2

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.

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Your Citation
Helmenstine, Anne Marie, Ph.D. "Predicting Formulas of Ionic Compounds." ThoughtCo, Aug. 25, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 25). Predicting Formulas of Ionic Compounds. Retrieved from Helmenstine, Anne Marie, Ph.D. "Predicting Formulas of Ionic Compounds." ThoughtCo. (accessed March 21, 2023).

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