The mass number is the total number of protons and neutrons (also total nucleon) in the nucleus.

See also Atomic Mass Unit, Atomic Mass.

The atomic mass is the mass of the naturally occurring mixture of isotopes of an element.

See also Mass Number, Atomic Mass Unit and Isotope.

(AMU) is equal to the mass of one-twelfth of a carbon-12 atom (6 protons + 6 neutrons (+ 6 electrons)), a 'very' little less than 1.66 x 10^-24 gram.

See also Mass Number and Atomic Mass.

Isobars are atoms with equal mass number but different atomic number.

Isotopes of a given element have the same number of protons but a different number of neutrons in the core. Isotopes have the same electronic structure as the given element and thus have a nearly identical chemical behavior. Larger elements tend to have more stable isotopes than smaller ones.
For example, I123, I125 and I131 are isotopes of iodine; they have a different mass number.