By definition, we have that the number of neutrons is given by:
# neutrons = atomic mass - # protons
We know that the atomic mass is equal to 84.
We know the atomic number of Krypton is equal to 36. This means that the Krypton has 36 protons and 36 electrons.
Therefore, substituting values we have:
# neutrons = 84 - 36
# neutrons = 48
Answer:
The Krypton has 48 neutrons