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

Explanation

Mass number `=81, i.e., p+n=81`
If protons `x`, then neutrons `=x+31.7/100xx x =1.317 x`
`:. x+1.317x=81` or `2.317x=81`
or `x=81/2.317=35`
Thus, protons `=35`, i.e., atomic `=35`
Hence, the symbol is `._(35)^(81)Br`.