Iodine (I) has an atomic number of 53, which means it has 53 protons in its nucleus. The isotope I131 has a mass number of 131, which means it has a total of 131 nucleons (protons and neutrons) in its nucleus.

To calculate the number of neutrons in I131, we can subtract the atomic number from the mass number:

Number of neutrons = Mass number - Atomic number

Number of neutrons = 131 - 53

Number of neutrons = 78

Therefore, there are 78 neutrons in atom I131.