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 + \(\frac{x\times 31.7}{100}\) = ( x + 0.317x)
Now, Mass no. of element = No. of protons + No. of neutrons
81 = x + (x + 0.317x)
= 2.317x
Or
x = 81/2.317 = 35
∴ No. of protons = 35, No. of neutrons = 81 – 35 = 46
Atomic number of element (Z) = Number of protons = 35
The element with atomic number (Z) 35 is bromine 8135Br.