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

Let the number of protons in the element be x.
$\therefore$ Number of neutrons in the element
= x + 31.7% of x
= x + 0.317 x
= 1.317 x
According to the question,
Mass number of the element = 81
$\therefore$ (Number of protons + number of neutrons) = 81
$\Rightarrow x+1.317x=812.317x=81x=\frac{81}{2.317}=34.95\therefore x=35$
Hence, the number of protons in the element i.e., x is 35.
Since the atomic number of an atom is defined as the number of protons present in its nucleus, the atomic number of the given element is 35.
$\therefore$ The atomic symbol of the element is ${}_{35}^{81}\text{Br}$