Take note that the nucleus of an atom is composed of protons and neutrons. And the number of particles present in the nucleus is referred as mass number (Also, called as atomic mass).
mass number = Number of Protons + Number of Neutrons
So, to determine the number of neutrons in atom, we only have to subtract the number of protons from the mass number.
# of neutrons = Mass Number - Number of Protons
To be able to apply this formula, refer to the atomic number and atomic weight of the elements. These two are listed in the Periodic Table of Elements.
Take note that atomic number refers to the number of protons in an atom of an element.
# of protons = atomic number
While, atomic weight is weighted average of the atomic masses of all the natural isotopes of an element. Its values in the Periodic Table are in decimal form. So to get the atomic mass, round off the atomic weight to its nearest whole number.
mass number = atomic weight rounded off to nearest whole number
For example, sodium's atomic number is 11 and its atomic weight is 22.989. So, sodium contains 11 protons and its mass number is 23.
Applying the formula above, the number of neutrons will be:
# number of neutrons = Mass Number - Number of Protons
# number of neutrons = 23 - 11
# number of neutrons = 12
Hence, there are are 12 neutrons in an atom of sodium.
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).
You find the atomic mass of an element by adding the neutrons and the protons. Therefore when you are looking for the neutrons you can instead subtract the protons from the atomic mass:
AM = P + N
N = AM - P
You find the protons by looking at the atomic number at the top and the atomic mass will be found on the bottom.
Let say you are using oxygen, the atomic number is 8 therefore there are 8 protons. The atomic mass is 16
N = 16 - 8
N = 8 therefore there are 8 neutrons.
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.