Question

The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of these elements is 40. What is the name given to such a pair of elements?
(A) Isotopes
(B) Isobars
(C) Both isotopes and isobars
(D) None of the above

Answer 1

Hint: Isotopes have the same number of protons so they have the same atomic number but different mass numbers. Isobars have the same mass number but different atomic number.

Complete solution step by step:
Isobars have the same number of nucleons. Nucleons are the sum of protons and neutrons present in the same nucleus. Isobars have different atomic numbers. Atomic number indicates number of protons and in a neutral atom, number of protons and electrons are the same. Isobars have different atomic numbers which indicates they have different numbers of protons and neutrons.
-Isobars are produced by beta decay, either a neutron is converted to proton or proton is converted to neutron is converted to proton so original and daughter nuclei are always isobars.
As atomic number of calcium is 20 it indicates number of protons are 20 and atomic number of argon is 18 so number of protons are 18.
As both have same mass number which indicates same number of nucleons i.e. Neutrons and protons, they are isobars.
The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of these elements is 40.

So (B) name is given to such a pair of elements.


Note: Isobar of cobalt is used in treatment of cancer. Isotopes have the same number of protons so they have the same atomic number but different mass numbers. isotones have the same number of neutrons. Isobars are produced by beta decay, either a neutron is converted to proton or proton is converted to neutron is converted to proton so original and daughter nuclei are always isobars.