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

We know that mass number of the element,

A = p+n = 81

Let the number of protons, p = x

Then, number of neutrons,

n= x+ 31.7 / 100x = 1.317 x

(As number of neutrons are 31.7% more than the protons.) Hence, from Eq. (i)

x + 1.317 x = 81

x = 81 / 2.317 = 34.958 = 35 (approx)

Therefore, number of protons = 35 and the symbol is ${}_{35}^{81}Br$ (Number of protons = Atomic number)