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

Solution:

Let the number of protons in the element be $x$.

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 $)=81$

$\Rightarrow x+1.317 x=81$

$2.317 x=81$

$x=\frac{81}{2.317}$

$=34.95$

$\therefore x=35$

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 ${ }_{35}^{81} \mathrm{Br}$