(i) they have the same atomic number but a different mass number.
(ii) they have the same number of electrons but a different number of neutrons.
(iii) they have the same number of neutrons but a different number of electrons.
(iv) the sum of the number of protons and neutrons is the same but the number of protons is different.
Correct Answer: (iv) the sum of the number of protons and neutrons is the same but the number of protons is different
Explanation: When the Sum of the number of protons and neutrons is equal but the number of protons is different, and these atoms are called isobars.