From the periodic table, you can calculate the number of neutrons in an atom by the following method:
- The atomic number (found on the periodic table) tells you the number of protons in the atom.
- The atomic mass (also found on the periodic table) represents the average mass of the atom's isotopes, which includes protons and neutrons.
- To find the number of neutrons, you round the atomic mass to the nearest whole number (called the mass number) and then subtract the atomic number from it.
Thus, the formula is:
Number of neutrons=Mass number (rounded atomic mass)−Atomic number\text{Number of neutrons}=\text{Mass number (rounded atomic mass)}-\text{Atomic number}Number of neutrons=Mass number (rounded atomic mass)−Atomic number
This method gives the approximate number of neutrons for the most common isotope of the element. Note that different isotopes of an element have different numbers of neutrons, so this is an average or approximate value for the element as found in the periodic table.
