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Predicting Formulas of Ionic Compounds Example Problem

Predicting the Molecular Formulas of Ionic Compounds

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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:
  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)
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 on 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. Nickle 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
Answer
  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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