Can deduce the number of neutrons by subtracting atomic number from mass number .
Number of neutrons can vary in an element, but number of protons is constant.
Is not the same as an element's atomic weight , which is the weighted mean of the masses of an element's constituent isotopes. Have the same number of protons, but different number of neutrons. Under natural conditions, elements occur as mixtures of isotopes. Different isotopes of the same element react chemically in same way. Some isotopes are radioactive.