The number of neutrons in an atom can be found by subtracting the atomic number from the atomic mass (or mass number). The atomic number is the number of protons and is found on the periodic table, usually at the top of the element's symbol. The atomic mass is the total number of protons plus neutrons in the nucleus. So,
Number of neutrons=Atomic mass−Atomic number\text{Number of neutrons}=\text{Atomic mass}-\text{Atomic number}Number of neutrons=Atomic mass−Atomic number
Since the atomic mass often is a decimal (due to isotopic averages), it is usually rounded to the nearest whole number when calculating the number of neutrons. For example, for oxygen with an atomic number of 8 and an atomic mass of about 16, the number of neutrons is approximately:
16−8=816-8=816−8=8
This method works for any element or isotope.
