To calculate the number of neutrons in an atom, you need two key pieces of information: the atomic number and the mass number.

• The atomic number is the number of protons in the atom and defines the element. It can be found on the periodic table.
• The mass number is the total number of protons and neutrons in the atom's nucleus.

The formula to find the number of neutrons is: Number of neutrons = Mass number – Atomic number For example, if an atom has an atomic number of 17 and a mass number of 35, the number of neutrons is: 35 (mass number) – 17 (atomic number) = 18 neutrons This calculation works because the mass number counts both protons and neutrons, so subtracting the protons (atomic number) leaves the number of neutrons