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)/(100) xx x = 1.317 x`
`:. x + 1.317 x = 81 or 2.317 x = 81 or x = (81)/(2.317) = 35`
Thus, Protons = 35, i.e., atomic no. = 35
Hence, the symbol is `._(35)^(81)Br`