Here's why:

* Atomic Number: The atomic number represents the number of protons in an atom's nucleus. This number defines the element.

* Iodine: Iodine is the element with atomic number 53.

* Iodine-131: The "131" refers to the mass number, which is the total number of protons and neutrons in the nucleus. The number of protons (atomic number) doesn't change for different isotopes of the same element.