Question

Which of the following pairs represents isobars?
A.${}_2^3He$ and ${}_2^4He$
B.${}_{12}^{24}Mg$ and ${}_{12}^{25}Mg$
C.${}_{19}^{40}K$ and ${}_{20}^{40}Ca$
D.${}_{19}^{40}K$ and ${}_{19}^{39}K$

Answer 1

Hint: The word isobar originally taken from the combination of Greek words-isos means equal and bar means weight. Isobars are different from isotopes.

Complete step by step answer:
We know that isos means equal and bar means weight in Greek. From this, it is clear that atomic weight is the same. The sum of the number of protons and neutrons together form the atomic mass. Isobars are atoms with different atomic numbers but the same mass number. The number in the left top of the element is atomic mass and the number in the left bottom is the atomic number. Since the atomic number should be different, that means the atoms are different. So, options (a), (b) and (d) can be eliminated where the atomic numbers are the same.
 So, the mass number is the same and the atomic number is different only in the option (c). Therefore, the correct answer is ${}_{19}^{40}K$ and${}_{20}^{40}Ca$.

Additional Information:
-The name ‘isobar’ was given by Alfred Walter Stewart in 1918.
-Since the sum of the number of protons and neutrons together form the atomic mass, we can also say atomic mass as the number of nucleons present in the nucleus.
-Isotopes are the atoms with the same atomic number but different mass number. Number of neutrons will be different in the isotopes.
-The term ‘isotope’ was coined by a Scottish doctor and writer Margaret Todd in 1913 in a suggestion to chemist Frederick Soddy.

Note: Don’t be confused with isotopes and isobars. Remember that isotope is related with atomic number (same) and isobar is related with atomic mass (same).