By J.R. Kambak – Updated August 30, 2022

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A chemical formula lists the elements that make up a compound, using the symbols from the Periodic Table. Each symbol tells you which element is present and, with a subscript, how many atoms of that element are in a single molecule. Subatomic particles—protons, neutrons, and electrons—constitute each atom. The atomic weight reflects the total number of protons and neutrons in the nucleus. Below we walk through how to count both the atoms and the subatomic particles for a typical formula, using calcium hydroxide, Ca(OH)₂, as an example.

Counting Atoms in the Compound

Step 1: Identify the Elements

Read the formula Ca(OH)₂ and spot the distinct elements: calcium (Ca), oxygen (O), and hydrogen (H).

Step 2: Determine the Number of Each Atom

Look for a subscript after each element symbol. If none is present, there is one atom of that element. In Ca(OH)₂, calcium appears without a subscript, so there is one Ca atom.

Step 3: Handle Polyatomic Ions

Polyatomic ions are groups of atoms that act as a single unit. They are enclosed in parentheses. The number outside the parentheses indicates how many of that ion are present. Here, “OH” is the ion, and the subscript 2 means there are two OH groups. Inside the ion, oxygen and hydrogen each have an implicit subscript of 1. Multiply by 2: 1 × 2 = 2 atoms of O and 1 × 2 = 2 atoms of H.

Summary: Ca – 1 atom; O – 2 atoms; H – 2 atoms.

Counting Subatomic Particles in the Atoms

Step 1: Retrieve Atomic Numbers and Weights

Use a reliable source such as the IUPAC Periodic Table. Calcium (Ca) has an atomic number of 20 and an atomic weight of 40.078. Oxygen (O) has 8 protons/electrons and a weight of 15.999. Hydrogen (H) has 1 proton/electron and a weight of 1.008.

Step 2: Compute Protons and Electrons

The atomic number equals the number of protons, and for a neutral atom the number of electrons is the same. Thus, Ca has 20 protons and 20 electrons; O has 8 protons and 8 electrons; H has 1 proton and 1 electron.

Step 3: Determine Neutrons

Neutrons are found by subtracting the atomic number from the mass number (the rounded atomic weight). For Ca, 40 – 20 = 20 neutrons. For O, 16 – 8 = 8 neutrons. Hydrogen has no neutrons in its most common isotope.

Step 4: Sum the Particles in Ca(OH)₂

Combine the counts from the individual atoms:

• Protons: 20 (Ca) + 16 (2 × O) + 2 (2 × H) = 38
• Electrons: 20 (Ca) + 16 (2 × O) + 2 (2 × H) = 38
• Neutrons: 20 (Ca) + 16 (2 × O) + 0 (2 × H) = 36

Things You’ll Need

• Reliable Periodic Table (e.g., IUPAC)
• Pen or pencil
• Paper
• Calculator (for quick arithmetic)