The number of ____ in an atom is called it ____ number. The relative mass of a ____ compared with a proton is one. The total number of protons and neutrons in an atom is called its ____ or mass number. Atoms of an element which have different numbers of neutrons are called ____. Isotopes have the same ____ properties as they contain the same number of ____, but their ____ properties can be different. These include ____ and some isotopes are radioactive.

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