Question

Consider the following statements ($X$ and $Y$ stand for two different elements)
(I) ${}_{32}{X}^{65}$ and ${}_{33}{Y}^{65}$ are isotopes.
(II) ${}_{42}{X}^{86}$ and ${}_{42}{Y}^{85}$ are isotopes.
(III) ${}_{85}{X}^{174}$ and ${}_{88}{Y}^{177}$ have the same number of neutrons.
(IV) ${}_{92}{X}^{235}$ and ${}_{94}{Y}^{235}$ are isobars.
The correct statements are

• A. II and IV only
• B. I, II and IV only
• C. II, III and IV only
• D. I, II, III and IV

Answer 1

The correct option is C II, III and IV only

Two atoms are said to be isotope if they have same atomic number but different mass number.
So, ${}_{42}{X}^{86}$ and ${}_{42}{Y}^{85}$ are isotopes.
Number of neutrons in ${}_{85}{X}^{174}$ $=174-85=89$
Number of neutrons in ${}_{88}{Y}^{177}$ $=174-85=89$
So both have same number of neutrons.
Two atoms are said to be isobars if they have different atomic number but same mass number.
So, ${}_{92}{X}^{235}$ and ${}_{94}{Y}^{235}$ are isobars.