Which one of the following elements will be an isobar of calcium if the atomic number of calcium is 20 and its mass number is 40?
Element with 20 protons and 18 neutrons
Element with 18 protons and 19 neutrons
Element with 20 protons and 19 neutrons
Element with 18 protons and 22 neutrons
Answer is Wrong!
Answer is Right!
This question was previously asked in
UPSC CDS-1 – 2018
Calcium (Ca) has atomic number (Z) = 20 and mass number (A) = 40.
An isobar of Calcium-40 must have a mass number of 40 but an atomic number different from 20.
Let’s examine the options:
A) 20 protons, 18 neutrons: Z=20, A=20+18=38. (Isotope of Ca)
B) 18 protons, 19 neutrons: Z=18 (Argon), A=18+19=37. (Argon-37)
C) 20 protons, 19 neutrons: Z=20, A=20+19=39. (Isotope of Ca)
D) 18 protons, 22 neutrons: Z=18 (Argon), A=18+22=40. (Argon-40). This element has a different atomic number (18) but the same mass number (40) as Calcium-40. Thus, Argon-40 is an isobar of Calcium-40.