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Consider the following statements ( $X$ and $Y$ stand for two different elements):
$(I)$ ${ }_{32}^{65} X$ and ${ }_{33}^{65} Y$ are isotopes.
$(II)$ ${ }_{42}^{86} X$ and ${ }_{42}^{85} Y$ are isotopes.
$(III)$ ${ }_{85}^{174} X$ and ${ }_{88}^{177} Y$ have the same number of neutrons.
$(IV)$ ${ }_{92}^{235} X$ and ${ }_{94}^{235} Y$ are isobars.
Which of the above statements are correct?
Only statements $II$ and $IV$ are correct
Only statements $I, II$ and $IV$ are correct
Only statements $II, III$ and $IV$ are correct
All statements are correct
Solution
(c)
For a nucleus, ${ }_Z^A X$
Mass number, $A=N+Z$
where, $N=$ number of neutrons
and $Z=$ number of protons.
In ${ }_{32}^{65} X$ and ${ }_{33}^{65} Y$, number of protons are different. So, these are not isotopes.
In ${ }_{42}^{86} X$ and ${ }_{42}^{85} Y$, number of protons are equal. So, these are isotopes.
In ${ }_{85}^{174} X$ and ${ }_{88}^{177} Y$, number of neutrons are $174-85=89$ and $177-88=89$.
So, both have same number of neutrons.
In ${ }_{92}^{235} X$ and ${ }_{94}^{235} Y$, both have same mass number, so these are isobars.
