To find the number of neutrons in an atom, you use a simple subtraction:

Number of neutrons=Mass number−Atomic number\text{Number of neutrons}=\text{Mass number}-\text{Atomic number}Number of neutrons=Mass number−Atomic number

What you need

  • Atomic number (Z)
    This is the number of protons. You get it from the periodic table (it’s usually the whole number above the element symbol).

  • Mass number (A)
    This is the total number of protons + neutrons for a specific isotope.

    • If the isotope is written like “Oxygen‑16” or 16O^{16}\text{O}16O, the 16 is the mass number.
* If you only have the periodic table, you can use the _atomic mass_ (the decimal number under the symbol), rounded to the nearest whole number, as an estimate of the mass number.

Step‑by‑step method

  1. Find the element on the periodic table.
    Note its atomic number ZZZ.
  1. Get the mass number AAA.
    • If an isotope is given (e.g., Carbon‑14, Potassium‑39), use that number directly.
 * If only the element is given (e.g., “oxygen”), round the atomic mass on the periodic table to the nearest whole number.
  1. Use the formula:

Neutrons=A−Z\text{Neutrons}=A-ZNeutrons=A−Z

This works because AAA counts protons + neutrons, and ZZZ counts just protons, so the difference is neutrons.

Quick examples

  • Oxygen‑16
    • Atomic number Z=8Z=8Z=8.
    • Mass number A=16A=16A=16.
    • Neutrons =16−8=8=16-8=8=16−8=8.
  • Just “oxygen” from the periodic table
    • Atomic number Z=8Z=8Z=8.
    • Atomic mass ≈ 15.999 → round to 16 → A≈16A\approx 16A≈16.
    • Neutrons ≈ 16−8=816-8=816−8=8 (an estimate because natural oxygen is a mix of isotopes).
  • Potassium‑39
    • From its name, A=39A=39A=39.
    • From the periodic table, potassium has Z=19Z=19Z=19.
    • Neutrons =39−19=20=39-19=20=39−19=20.

Why there’s sometimes “≈”

In real samples, elements often exist as a mixture of isotopes , so the periodic table shows an average atomic mass (a weighted average), not a single exact mass number.

  • When you round this average to get AAA, your neutron count is an estimate for the most common isotope, not a perfect value for every atom of that element.

One‑line reminder

To find the number of neutrons in an atom, take the (mass number or rounded atomic mass) and subtract the atomic number: neutrons =A−Z=A-Z=A−Z.

Information gathered from public forums or data available on the internet and portrayed here.