Iodine-131, a radioisotope typically employed in the diagnosis of thyroid disorders, has a mass number, $A$, of 131 and an atomic number, $Z$, of 53 . The number of neutrons, $N$, of iodine-131 is:
A. 131
B. 78
C. 53
D. 25
Choose A, B, C or D as your solution.
The correct answer is $\mathrm{B}$. The number of neutrons is the mass number $A$ minus the atomic number $Z$. Thus, the number of neutrons of iodine-131 is $N=A-Z=$ $131-53=78$