Atoms with the same atomic number but different neutron numbers, and hence different mass numbers, are known as isotopes.
ˈæɾəmz wɪððə sˈeɪm ɐtˈɑːmɪk nˈʌmbɚ bˌʌt dˈɪfɹənt nˈuːtɹɑːn nˈʌmbɚz, ænd hˈɛns dˈɪfɹənt mˈæs nˈʌmbɚz, ɑːɹ nˈoʊn æz ˈaɪsətˌoʊps.