By David Chandler
Updated Aug 30, 2022

Emilija Randjelovic/iStock/GettyImages

Chemical formulas give a concise snapshot of the elements that compose a compound and the number of atoms of each. A molecular formula lists each element’s symbol followed by a subscript that denotes how many atoms of that element are present. When a single atom is present, the subscript is omitted by convention. Polyatomic groups—such as NH₄⁺ or SO₄^2‑—are enclosed in parentheses, and a subscript after the closing parenthesis indicates how many of that group are included.

Step 1: Identify the Elements

Read the formula and write down every unique element symbol. For example, the ammonium phosphate formula (NH₄)₃PO₄ contains nitrogen (N), hydrogen (H), phosphorus (P), and oxygen (O).

Step 2: Assign a Subscript of One Where Needed

Any element that lacks an explicit subscript is understood to have a single atom. Explicitly adding a subscript of one—especially during a manual count—helps avoid overlooking that atom.

Step 3: Handle Polyatomic Groups

Sum the atoms inside the parentheses, then multiply by the subscript that follows the closing parenthesis. In (NH₄)₃, the group contains 1 N + 4 H = 5 atoms; multiplied by 3 gives 15 atoms contributed by the ammonium groups.

Step 4: Count Atoms Outside Parentheses

For the remaining part of the formula—PO₄ in our example—add the subscripts directly: 1 P + 4 O = 5 atoms.

Step 5: Calculate the Total

Combine the results from the grouped and ungrouped sections. For ammonium phosphate, the total number of atoms is 15 + 5 = 20.

Things Needed

• Pencil
• Paper
• Molecular formula of the compound you’re analyzing