Whats the mass number of the atom 86/36 Kr?

Answer: 83

Explanation: When given a nuclear symbol (86/36 Kr), the number on the top left of the element symbol will be the mass number.

Answer: 86 is the mass number

Explanation: Although this example is an isotope, since the actual mass number of Kr is just 84. But for this sample the mass number is 86, it is the number of protons and neutrons present in the nucleus of Kr. While 36 is the atomic number and is indicated by the number of electrons present in the atom of Kr.