How many neurons does elements X have if's atomic number is 21 and its mass number is 76

The element has 55 neutrons.

Explanation:

mass number = A = 76

atomic number = Z = 21

Mass number is the sum of number of protons and neutrons present in a nucleus while as the atomic number is the number of protons present in a nucleus.

so the number of neutrons = Mass number - Atomic number

n = A-Z

n=76-21

n = 55