Answer:
120 neutrons
Step-by-step explanation:
Mercury has an atomic number of 80. The atomic number (Z) represents the number of protons. Then,
Z = p⁺ = 80
Mercury has a mass number of 200. The mass number (A) represents the sum of protons and neutrons. Then,
A = p⁺ + n° = 200
We can establish that,
A = Z + n°
n° = A - Z
n° = 200 - 80 = 120