How do you find the isotope of an element in an atom?

A. Like was said earlier find 19 on the Periodic table then look up that element either on the Internet (normally common isotopes and 1/2 lives will be listed) or go to a table of isotopes (extended periodic table) to see the number of neutrons. For an individual isotope, the number of neutrons can be found by subtracting it's atomic number (number of protons) from it's atomic m number (number of protons + neutrons) This should not be confused with the relative atomic m which is the average abundance atomic m number of the differing isotopes found. For example Zinc Atomic number 30, standard atomic weight ~ 65, you would ume 30 protons and 35 neutrons, but Zinc 65 is very unstable and not found in nature (Half-life of 244 days) There is a lot of Zinc 64 and some Zinc 66 and Zinc 67 making the atomic weight approximately 65.