Calcium and Argon
Explanation
Isobars are atoms of different chemical elements that possess the same mass number (A) but different atomic numbers (Z). Consequently, they have a different number of protons and electrons but the same total number of nucleons (sum of protons and neutrons).
Analysis of Options:
- Hydrogen and Deuterium Hydrogen ($^1_1\text{H}$) and Deuterium ($^2_1\text{H}$): Both have the same atomic number ($Z=1$) but different mass numbers. These are Isotopes of Hydrogen.
- Carbon-12 and Carbon-14 Carbon-12 ($^{12}_6\text{C}$) and Carbon-14 ($^{14}_6\text{C}$): Both have the same atomic number ($Z=6$) but different mass numbers. These are Isotopes of Carbon.
- Calcium and Argon Calcium ($^{40}_{20}\text{Ca}$) and Argon ($^{40}_{18}\text{Ar}$):
- Calcium: Atomic number ($Z$) = 20, Mass number ($A$) = 40. (Protons = 20, Neutrons = 20).
- Argon: Atomic number ($Z$) = 18, Mass number ($A$) = 40. (Protons = 18, Neutrons = 22).
- Since they have different atomic numbers but the same mass number, they are Isobars.
- Chlorine-35 and Chlorine-37 Chlorine-35 ($^{35}_{17}\text{Cl}$) and Chlorine-37 ($^{37}_{17}\text{Cl}$): Both have the same atomic number ($Z=17$) but different mass numbers. These are Isotopes of Chlorine.
Key Takeaway:
The primary distinction is that Isotopes belong to the same element (same $Z$, different $A$), whereas Isobars belong to different elements (different $Z$, same $A$).