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?
[amp_mcq option1=”Element with 20 protons and 18 neutrons” option2=”Element with 18 protons and 19 neutrons” option3=”Element with 20 protons and 19 neutrons” option4=”Element with 18 protons and 22 neutrons” correct=”option4″]
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.