An element with mass number `81` contains `31.7%` more neutrons as compared to protons. Write the symbol for the isotope in the standard fromat.
Strategy: Let us assume that the number of protons `=p`. Then, Number of neutrons `(n) = p +31.7%`of `p`

Explanation

Mass number `=` Number of protons `+` Number of neutrons
`81=p +n`
or `p+(p+(31.7)/(100)xxp)=81`
`p+(p +0.317p) = 81`
`p+p(1+0.317) = 81`
`p+1.317p = 81`
`(1+1.317)p = 81`
`2.317 p = 81`
`p = (81)/(2.317) 34.95~= 35`
According to the periodic table, the element with atomic number `35` is bromine, Br. Therefore, the symbol is `_(35)^(81)Br`.