How can someone figure out the number of neutrons in a nucleus?

Question

i know hydrogen has no neutrons, but for everything else, how is the number of neutrons determined?

can this be determined with the knowledge of the atomic number?

Answer 1

just subtract the atomic number from the atomic weight.  you may have to know isotopes so understand that that number may vary.  you can discount electrons as they mass 2000 times less than a proton.

Answer 2

To get the number of neutrons:

Atomic mass - Atomic number = # of neutrons

Example:

deuterium has an atomic number of 1 and an atomic mass of 2

2 - 1 = 1,  so deuterium has 1 neutron