The Hydroxide Ion and Ionic Compounds

Context

The ionic compounds discussed thus far have been made from monotomic ions, ions made from single atoms. Ionic compounds can also be formed from polyatomic ions.

What is the hydroxide ion?

The hydroxide ion is a particle with a negative one (-1) charge that has two atoms: one oxygen atom and one hydrogen atom. The formula for the hydroxide ion is OH^-. It is an example of a simple polyatomic ion.

Explanation

Polyatomic ions are charged particles that have two or more atoms that are connected and act as one unit. Some common polyatomic ions are NH₄⁺, NO₃^-, SO₄^2-, and CO₃^2-.

This section will help you:

• Learn how to determine formulas for ionic compounds using polyatomic ions.
• Understand the significance of the hydroxide ion.
• Learn some of the properties of bases.

Model

Each polyatomic ion has a charge associated with it that doesn't change. The charges on the examples above don't change. These polyatomic ions can be combined with the elements we have identified as forming salts or with other polyatomic ions to form ionic compounds. It is always the case, however, that the charges of the ions should balance out: all electrons must be accounted for. Here are a some examples: NH₄Cl, (NH₄)₂SO₄, NaNO₃.

The hydroxide ion (OH^-) is an important ion because an aqueous (water) solution is determined to be basic (as opposed to acidic) if there are extra hydroxide ions in the solution. Many bases are ionic compounds that have OH^- as the negative ion. NaOH, sodium hydroxide, is a good example of a base. When NaOH is placed in water it separates into Na⁺ and OH^- ions that are free to float around in the solution. Any of the elements in the first two columns (excluding hydrogen) combined with the hydroxide ion will form an ionic compound that will be a base. Some examples: LiOH, Mg(OH)₂, etc.

Thinking Questions

1. Do polyatomic ions always have a charge?
2. Why is the hydroxide ion special?