Mass numberĀ is theĀ numberĀ of protons and neutrons in anĀ atom, and it tells us about theĀ massĀ of theĀ atomĀ in amu, orĀ atomic massĀ units.Ā Atomic massĀ is the averagemassĀ of all the isotopes of a certain type. It is a weighted average that takes into account the abundances of all of the different isotopes.The full chemical symbol for an element shows itsĀ mass numberĀ at the top, and its atomic numberĀ at the bottom. This symbol tellsĀ youĀ that the chlorineĀ atomĀ has 17 protons. It will also have 17 electrons, because theĀ numberĀ of protons and electrons in anĀ atomĀ is the same.The atomic number specifies how many protons an element has in its nucleus. In the periodic tableĀ of the elements, elements are arranged in order of ascending atomic number. Isotopes are atoms that have the same number of protons but different numbers of neutrons.
