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`.