To calculate the number of neutrons in an atom, you subtract the atomic number from the mass number.

Core idea in one line

Number of neutrons = mass number − atomic number.

  • Atomic number (Z) : Number of protons in the nucleus; this is the number you see on the periodic table for each element.
  • Mass number (A) : Total number of protons + neutrons in the nucleus; often given as a whole number or implied by the common isotope (like carbon‑12, oxygen‑16).
  • Neutrons (N) : N = A − Z.

Quick example

Imagine you’re “meeting” a carbon atom like a character in a short story:

Carbon‑12 walks into the lab with a badge that says “6” on one side and “12” on the other. The 6 is its atomic number (protons), the 12 is its mass number. The rest of its mass “weight” must come from neutrons.

For carbon‑12:

  • Atomic number Z = 6 (so 6 protons).
  • Mass number A = 12.
  • Neutrons N = 12 − 6 = 6.

If you only know the atomic number

If you’re given only the atomic number and nothing else, you cannot uniquely determine the exact neutron count, because:

  • Many elements have multiple isotopes, meaning different possible numbers of neutrons.
  • Example: Carbon can be carbon‑12 (6 neutrons), carbon‑13 (7 neutrons), carbon‑14 (8 neutrons), all with the same atomic number 6.

In that case, you can:

  • State that you need the mass number (or at least the isotope name, like “carbon‑13”).
  • Or, if the question is about the most common isotope, you look up that isotope’s mass number and then do A − Z.

Another worked example

For oxygen‑16:

  • Atomic number Z (oxygen) = 8.
  • Mass number A = 16.
  • Neutrons N = 16 − 8 = 8.

For chlorine‑35:

  • Z = 17.
  • A = 35.
  • N = 35 − 17 = 18.

HTML table of examples

Below is a small HTML table showing a few isotopes and their neutron counts:

html

<table>
  <thead>
    <tr>
      <th>Element (isotope)</th>
      <th>Atomic number (Z)</th>
      <th>Mass number (A)</th>
      <th>Neutrons (N = A − Z)</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Hydrogen‑1</td>
      <td>1</td>
      <td>1</td>
      <td>0</td>
    </tr>
    <tr>
      <td>Carbon‑12</td>
      <td>6</td>
      <td>12</td>
      <td>6</td>
    </tr>
    <tr>
      <td>Carbon‑14</td>
      <td>6</td>
      <td>14</td>
      <td>8</td>
    </tr>
    <tr>
      <td>Oxygen‑16</td>
      <td>8</td>
      <td>16</td>
      <td>8</td>
    </tr>
    <tr>
      <td>Chlorine‑35</td>
      <td>17</td>
      <td>35</td>
      <td>18</td>
    </tr>
  </tbody>
</table>

Quick TL;DR

  • Use: Number of neutrons = mass number − atomic number.
  • You must know the mass number (or isotope) to get an exact neutron count.

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