Let the number of protons in the element be x.
`:.` Number of neutrons in the element
`= x + 31.7%` of `x`
`= x + 3.317 x`
`= 1.317 x`
According to the question,
Mass number of the element = 81
`:.` (Number of protons + number of neutrons) = 81
`rArr x + 1.317 x = 81`
`2.137 x = 81`
`x = (81)/(2.317)`
`= 34.95`
`:. 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.
`:.` The atomic symbol of the element is `._(35)^(81)Br`
