2.42    An element with mass number 81 contains 31.7% more neutrons as compared to protons. Assign the atomic symbol.

D Divya Prakash Singh

Let the number of protons of an atom be $'p'$.

Then the number of neutrons will be,

$p + \frac{31.7}{100}p = 1.317p$

and the mass number is $81$ (Given).

Mass number = number of neutrons + number of protons.

Therefore,

$81 = 1.317p + p$

$\Rightarrow p = \frac{81}{2.317} =35$

Thus, there are $35$ numbers of protons which is also its atomic number.

Hence, the symbol for the element is $_{35}^{81}\textrm{Br}$.

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