Explain how you can determine the number of neutrons in an atom

Answer 1

# protons = atomic number--that whole number usually in bold black print on the periodic table.
# protons + # neutrons = mass number--that isn't on the periodic table but usually is in the problem.
#p + #n = mass # and rearrange to

For example, 17Cl35 is chlorine with 17 protons and a total of 35 nucleons.(35 nucleons is another way of saying the mass number is 35)
35-17 = 18 neutrons.
#n = mass # - #p