उत्तर : 1. "समान भार संख्याएँ किंतु विभिन्न परमाणु संख्याएँ"
व्याख्या :

Nuclei are classified based on the number of protons (atomic number) or the total number of nucleons (mass number):

Isotopes: Atoms of an element which have the same atomic number but different mass numbers are called isotopes. All isotopes have similar chemical properties.

Isobar: Nuclei which have the same mass number (A) but a different atomic number (Z) are called isobar.

Even-neutronic: Nuclei having equal number of neutrons are called even-neutronic. For them both atomic number (Z) and mass number (A) are different, but the value of (A – Z) is the same.