Substance with same atomic number but different mass number

Ans. a (Isotope). (Ref. Radioactive isotope in medicine and biology by Quimby, 2nd ed., 26)TermDefinitionIsotopesAtoms having same atomic numbers, different mass numbers (hence different neutron number) e.g., 17CI35, 17CI37 (same no.of protons and electrons)IsobarsAtoms having same mass numbers, different atomic number (hence different neutron number) e.g., 28Ni64, 30Zn64IsotonesAtoms have same neutron number, different atomic mass number e.g., 18A40IsomersAtoms having same atomic and mass numbers but different energy states in nucleus.