Why Does Calcium Hydroxide Have Parentheses in Its Formula?

Calcium hydroxide’s formula is Ca(OH)₂. The parentheses are there because hydroxide, OH⁻, is a group of atoms that acts as one ion, and the formula needs two of those ions for each calcium ion.

The number outside the parentheses applies to everything inside them. So Ca(OH)₂ contains one calcium atom, two oxygen atoms, and two hydrogen atoms. The parentheses are part of the way the formula is written; they do not mean that calcium hydroxide is a molecule with a special bracketed structure.

The ions and their charges

Calcium forms a positively charged ion, Ca²⁺. Hydroxide is a negatively charged polyatomic ion: one oxygen atom and one hydrogen atom together have a charge of −1, written OH⁻.

An ionic compound must have an overall charge of zero. One Ca²⁺ ion has a charge of +2, so it takes two OH⁻ ions, each with a charge of −1, to balance it:

$$(+2) + 2(-1) = 0$$

That gives one calcium ion for every two hydroxide ions. The formula is therefore Ca(OH)₂.

What the parentheses and subscript mean

Read the formula in pieces:

• Ca means one calcium atom, represented in the solid by calcium ions.
• (OH) identifies the hydroxide ion as a group.
• ₂ means there are two hydroxide groups.

The subscript outside the parentheses multiplies each atom count inside them. Thus, (OH)₂ means two O atoms and two H atoms. The formula contains three elements’ atom counts: Ca₁O₂H₂.

Parentheses are needed when a polyatomic ion occurs more than once. If there is only one hydroxide ion, the formula is written without parentheses: sodium hydroxide is NaOH. If there are three hydroxide ions, as in aluminum hydroxide, the formula is Al(OH)₃.

Compound	Ions in the formula unit	Formula
Sodium hydroxide	One Na⁺ and one OH⁻	NaOH
Calcium hydroxide	One Ca²⁺ and two OH⁻	Ca(OH)₂
Aluminum hydroxide	One Al³⁺ and three OH⁻	Al(OH)₃

Worked example: writing the formula from the ions

Suppose you are asked to write the formula for a compound made from calcium ions and hydroxide ions.

1. Write the ion symbols and charges: Ca²⁺ and OH⁻.
2. Choose counts that make the total charge zero. One calcium ion contributes +2; two hydroxide ions contribute −2.
3. Write one Ca and two hydroxide groups. Because OH is a polyatomic ion and appears twice, put it in parentheses and add the subscript outside: Ca(OH)₂.

The formula is an empirical ratio for the ionic solid, not a claim that each piece is a separate Ca(OH)₂ molecule. Ionic solids are described in terms of formula units—the smallest whole-number ratio of their ions.

You can also check the atom count or calculate the molar mass. Using approximate atomic masses Ca = 40.078, O = 15.999, and H = 1.008 g/mol:

$$40.078 + 2(15.999 + 1.008) = 74.092\text{ g/mol}$$

The factor of 2 applies to both the oxygen and hydrogen in OH, just as it does when counting atoms.

Common mistakes

• Writing CaOH₂: This notation puts the 2 only on H. It does not show two hydroxide ions and gives the wrong atom counts for calcium hydroxide.
• Writing Ca₂OH: A subscript applies to the element or group immediately before it; this would indicate two calcium atoms, not two hydroxide ions.
• Putting parentheses around a group only once: Parentheses are not needed for a single OH group, so sodium hydroxide is NaOH, not Na(OH).
• Treating parentheses as evidence of a molecule: Calcium hydroxide is an ionic solid. The parentheses show how to count a repeated polyatomic ion in its formula.
• Thinking the 2 means two hydroxide atoms: The subscript counts entire OH groups. Each group has one O and one H, so two groups contain two of each.

The same notation works for other polyatomic ions. For example, magnesium nitrate is Mg(NO₃)₂: the 2 counts two complete nitrate ions, including two N atoms and six O atoms.