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

Explanation

An element can be identified by its atomic number only. Let us find the atomic number.
Let the number of protons = x
`:.` Number of neutrons `=x+(x xx31.7 )/(100) =(x + 0.317 x)`
Now, Mass no. of element = No. of protons + No. of neutrons
`81 = x+x+0.317 x = 2.317 x or x=(81)/(2.317)=35`
`:. " "` No. of protons = 35, No. of neutrons = 81 -35 = 46
Atomic number of element (Z) = No. of protons = 35
The element with atomic number (Z) 35 is bromine `._(35)^(81)Br`.