Question -
Answer -
(i) Atomic number
The atomic number of an element is the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.
(ii) Mass number
The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of boron has 5 protons and 6 neutrons. So, the mass number of boron is 5 + 6 = 11.
(iii) Isotopes
(iv) Isobars
Isobars are atoms having the same mass number, but different atomic numbers i.e., isobars are atoms of different elements having the same mass number.
For example, 
and
areisobars.Two uses of isotopes are:
(i) One isotope of uranium is used as a fuel in nuclear reactors.
(ii) One isotope of cobalt is used in the treatment of cancer.