Concept:Isobars are atoms of different elements sharing the same mass number but having different atomic numbers.
Explanation:Isobars have the same total number of nucleons (protons + neutrons), so both mass number and total number of nucleons are identical.
The atomic number, which counts the protons in the nucleus, is always different because isobars belong to different elements.
Since atomic number changes, the number of electrons in a neutral atom also changes, but atomic number is the fundamental property that never stays the same.
For example, Carbon-14 (
6 protons,
8 neutrons) and Nitrogen-14 (
7 protons,
7 neutrons) are isobars with distinct atomic numbers.
Therefore, the property that is always different for isobars is the atomic number.
Answer:Atomic number (Option D).