What does the number 84 in the name krypton-84 represent?
The number of nucleons (i.e. protons + neutrons)
Depending on the type of periodic table you have, you may have noticed, that every element is equipped with two numbers. Take for instance your krypton-84:
The number on top tells you how many nucleons (protons + neutrons) you have (this usually, also adds up to the molar mass of this specific isotope). The bottom number tells you how many protons you have in the nucleus of this element.
In the case of Krypton-84, this means that you have 84 nucleons, where 36 of these are protons, and the remaining 48 are neutrons.