● Isotopes are the atoms of same chemical element having different
atomic mass. This means they differ in neutron number. For ex- 17Cl35 and 17Cl37. Here both atoms are chlorine atoms and have atomic number 17. So, each atom has 17 electrons and 17 protons. But the former one has atomic mass 35 and the latter one has atomic mass 37. This implies that former one has 18 neutrons (35-17) and latter one has 20 neutrons (37-17). Hence, they are isotopes.
● Isobars are the atoms of different elements having same atomic mass.
This implies that the sum of protons and neutrons in each atom is same. For ex- 18Ar40 and 20Ca40. Here, the former one is argon atom and latter one is calcium. Their atomic numbers are different. But their atomic mass is same i.e. 40. This implies that the number of nucleons in both atoms is same, even though the proton, electron and neutron numbers are different.
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.