Question

Isotopes differ in:
$A.$ Valence electron
$B.$ Chemical properties
$C.$ Atomic number
$D.$ Mass number

Answer 1

An isotope is defined as the element that has the same atomic number (i.e. it has the same number of protons and the same number of electrons. In isotopes the number of neutrons present in the nucleus is not the same..

Complete solution:
For better understanding of isotopes let us first consider few example of isotopes, $_6^{12}C$ or $_6^{13}C$ and $_1^1H$ , $_1^2H$ or $_1^3H$ are the examples isotopes of carbon and hydrogen respectively, In carbon isotopes they have same atomic number that is $6$ but have different mass number that is $12$ and $13$ respectively. We know the mass number is equal to the sum to number of neutron and proton as they have same atomic number so they have same number of proton but the mass number is different of these isotopes because they contain different number of neutron in $_6^{12}C$number of neutron is $6$ while in $_6^{13}C$number of neutron is $7$ . Similarly in the case of isotopes of hydrogen the number of neutron in all the isotopes of hydrogen is different that is in $_1^1H$ number of neutron is $0,$ in $_1^2H$ number of neutron is $1$ and in $_1^3H$ number of neutron is $2$ respectively. Thus, we may conclude that isotopes differ only in mass due to the presence of different numbers of neutrons.

Hence the correct option is (D).

Note: It is to be noted that isotopes differ only by mass number but they have similar chemical properties because we know that the number of electrons present in an atom is the same so their electronic configuration remains the same, as the electronic configuration is the same so its chemical properties remain the same.