The number "84" in the krypton-84 indicates the mass number of this particular isotope, which is the sum of protons and neutrons in the nucleus. It is also approximately equal to the atomic weight of the isotope.
Since each isotope of krypton has 36 protons, equal to the atomic number of the element, the mass number reflects the number of neutrons. Krypton-84, for example, has 84 neutrons in its nucleus. This isotope of krypton is the most abundant naturally occurring form of the element, found approximately 57 percent of the time. It is a stable isotope that has no half-life and does not decay into other elements over time. Unstable isotopes of the elements rubidium, strontium and bromine decay into krypton-84 over time.