Question

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

Answer 1

Let the number of protons in the element be x.
∴ Number of neutrons in the element = x + 31.7% of x = x + 0.317 x = 1.317 x
According to the question, the Mass number of the element = 81
∴ (Number of protons + number of neutrons) = 81
⇒ x + 1.317 x = 81
2.317 x = 81x = \(\frac{81}{2.317}\) = 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 \(_{35}^{81}\textrm{Br}\).