In the name "Krypton 84," the number 84 represents the mass number of the isotope of krypton being referred to. Krypton is an element that can have several isotopes, which are atoms with the same number of protons but different numbers of neutrons.
The mass number of an isotope is the sum of its protons and neutrons. In this case, Krypton 84 refers to the isotope of krypton with a mass number of 84. It means that this particular isotope has 36 protons (since krypton has an atomic number of 36) and 48 neutrons.
Isotopes of an element may have different physical properties but generally exhibit similar chemical behavior due to their shared number of protons.