How are Iodine 131 and Iodine 127 sub-atomically the same and sub-atomically different?
a. They each have 53 protons and 53 neutrons
b. they have the same number of protons and different number of neutrons.
c. they have the same number of electrons and different number of protons.
d. they have the same number of protons and different number of electrons.