The atomic number is the number of protons in the nucleus . The number of protons determines the type of element but protons and neutrons make up the weight. Isotopes have the same atomic number but different mass numbers OR, isotopes have the same number of protons but different numbers of neutrons. Chlorine, atomic number 17, has isotopes with 18 and 20 neutrons. This gives the chlorine isotopes the mass numbers 35 and 37.
Most elements have more than one naturally occurring isotope.
Only unstable isotopes are radio-active. Mass numbers ignore the mass of electrons the difference in the mass of protons and neutrons the very small change in mass when protons and neutrons form a nucleus. The approximate relative mass of an element is the average of the mass numbers of its isotopes. The tutorial has as gentle as possible an introduction to this difficult topic. It has a lot of interactions to help build up your confidence.