To calculate the number of neutrons in an atom, you have to either memorize the formula, or better yet, understand how different numbers on the periodic table are derived so you can calculate it from there.
The formula for finding out the number of neutrons in an atom is atomic mass – atomic number. The intuition behind this hinges on how the atomic mass presented on the periodic table is calculated. The atomic mass of an atom is determined by only the number of protons and neutrons. Even though subatomic particles includes electrons as well, the mass of electrons is so insignificant that they are essentially negligible in this calculation. You also have to know that the atomic number of an element on the periodic table to equal to the number of protons that atom contains. It then makes perfect sense why the number of neutrons is equal to the atomic mass (protons + neutrons) minus the atomic number (protons only).
On the periodic table you are given the atomic number (top number) and the mass number (bottom number) of each element. The mass number is the sum of protons and neutrons in the nucleus. The atomic number is the number of protons. Therefore, you can subtract the atomic number from the mass number to find the number of neutrons.
For example, Carbon’s atomic number/number of protons is 6 and the mass number is 12.011. This means there are 6 neutrons (approximately).
The number of neutrons can be calculated by simply looking at the Periodic Table of Elements.
The number of neutrons= mass number – atomic number
Each element in the table has the mass number (atomic weight) located directly under the Element name and the atomic number is located at the top left hand corner of an element in the table.
Don’t forget to round the mass number (atomic weight) to the nearest whole number.