If you are given only the mass number of neutrons, how do we find an atomic number?

You can find the atomic number by subtracting the atomic mass by the number of neutrons.

Mass number is the sum of protons and neutrons.

Atomic number is the total number of protons present.

Mass number = protons + neutrons

Atomic number = protons present.

From the above equations we can conclude that,

Atomic number = Mass number - neutrons.

Thus we can find out the atomic number by subtracting the number of neutrons from the mass number.