By Contributing Writer • Updated August 30, 2022

Atoms bond by sharing electrons, and knowing the electron count of a compound is the first step to understanding its chemistry. Using the periodic table and basic arithmetic, you can determine how many electrons are present in any molecule or ion.

Step 1: Identify the Elements and Their Quantities

Write down every element in the chemical formula and note how many atoms of each appear. For example:

• KNO₃ – Potassium (1 atom), Nitrogen (1 atom), Oxygen (3 atoms)
• SO₄^2‑ – Sulfur (1 atom), Oxygen (4 atoms)

Step 2: Retrieve Atomic Numbers

Consult a reliable periodic table (e.g., the PubChem Periodic Table) to find each element’s atomic number—the integer directly above its symbol. In the examples above: K = 19, N = 7, O = 8, S = 16.

Step 3: Compute the Electron Sum

Multiply each element’s atomic number by its atom count, then add the products:

• KNO₃: (19×1) + (7×1) + (8×3) = 50 electrons
• SO₄^2‑: (16×1) + (8×4) = 48 electrons (before charge adjustment)

Step 4: Adjust for Ionic Charge

For ions, modify the total from Step 3 by the charge value: add the charge for negative ions, subtract for positive ions, and leave unchanged for neutral species. The sulfate ion carries a 2‑ charge, so 48 + 2 = 50 electrons.

Tools You’ll Need

• Scientific calculator (or spreadsheet)
• Access to an up‑to‑date periodic table

References

• Whitten, K.W.; Davis, R.E.; Peck, L.; Stanley, G.G. Chemistry, 2009.