The identity of an element is defined by its number of _______________. For example, every atom of Carbon has ___ protons and only atoms with 6 protons are __________. However, atoms of the same element can have ________________ number of neutrons. When atoms of the same element have different numbers of neutrons they are called ________________ . Since the mass of an atom is determined by the total number of protons and neutrons, isotopes of the same element will have different __________ despite having the _________ numbers of protons. Also, isotopes of the same element will have similar _____________ behavior. This is because chemical _____________ is based on the number of ____, and isotopes only have different numbers of ____.