You calculate how many neutrons an atom has by subtracting its atomic number from its mass number.

The core formula

For any atom or isotope, use: Neutrons = Mass number − Atomic number

  • Atomic number (Z): Number of protons; this tells you which element it is. You get this from the periodic table.
  • Mass number (A): Total number of protons + neutrons for a specific isotope (often written in the name, like carbon‑14, or as a superscript).

So neutrons = A − Z.

Step‑by‑step method

  1. Find the element’s atomic number on the periodic table (Z).
  2. Get the mass number:
    • If an isotope is given (like oxygen‑16, K‑39), the number in the name is the mass number A.
 * If only the element is given (like just “oxygen”), round the atomic mass on the periodic table to the nearest whole number to estimate A.
  1. Subtract: neutrons = A − Z.

Quick examples

  • Oxygen‑16:
    • Z (oxygen) = 8
    • A = 16
    • Neutrons = 16 − 8 = 8
  • Potassium‑39:
    • Z (potassium) = 19
    • A = 39
    • Neutrons = 39 − 19 = 20
  • Neon‑22:
    • Z (neon) = 10
    • A = 22
    • Neutrons = 22 − 10 = 12

Isotopes twist (same element, different neutrons)

Atoms of the same element always have the same number of protons (same Z) but can have different numbers of neutrons, giving different isotopes.

Example with chlorine:

  • Cl‑35: Z = 17, A = 35 → neutrons = 18
  • Cl‑37: Z = 17, A = 37 → neutrons = 20

This is why isotopes of one element can have different mass numbers but still be the same element.

Tiny “real life” tip

If you’re reading a periodic table in class or online and only see:

  • Top number: atomic number Z
  • Bottom number: atomic mass (decimal)

You can:

  • Round the atomic mass to the nearest whole number to estimate A, then do A − Z to get an approximate neutron count.

TL;DR: Look up the atomic number, find or estimate the mass number, then do mass number minus atomic number to get neutrons.