Question

One isotope of a non-metallic element has mass number $127$ and $74$ neutrons in the nucleus. The anion derived from the isotope has $54$ electrons. Hence, symbol for the anion is :

• A. ${}_{54}^{127}{\text{X}}^{-}$
• B. ${}_{53}^{127}{\text{X}}^{-}$
• C. ${}_{53}^{74}{\text{X}}^{-}$
• D. ${}_{54}^{74}{\text{X}}^{-}$

Answer 1

The correct option is B ${}_{53}^{127}{\text{X}}^{-}$

No. of protons or electrons in $X$ are $127-74=53$
So, symbol of anion is ${}_{53}^{127}{\text{X}}^{-}$.