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. 2 Li⁺ ions are required to balance 1 O^2- ion
2. 1 Ni ²⁺ ion is required to balance 1 S^2- ion
3. 1 Bi³⁺ ion is required to balance 3 F^- ions
4. 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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