Question

An element with a mass number of 81 contains 31.7% more neutrons as compared to protons. Identify the element.
A. Sc
B. Ba
C. Br
D. I

Answer 1

Hint: Assume the number of protons and then find the number of neutrons according to the relation given in the question. After that add both the values to obtain mass number (total number of neutrons and protons in the nucleus make up the mass number of an element) and finally find the atomic number to identify the particular element.

Complete answer:
To solve the question, let the number of protons be ‘x’.
According to the question, there are 31.7% more neutrons than protons, so, the account of neutrons are$\dfrac{31.7\text{x}}{100}$ of protons plus the protons. The total number of neutrons will be $\left[ \left( \dfrac{31.7\text{x}}{100} \right)+\text{x} \right]$ which is 1.317x . As, the mass number of elements is given as 81.
Mass number is actually the sum of protons and neutrons of an element. Mass number = no. of protons + no. of neutrons. The number of neutrons are 1.317 and protons are x. So, 81 is equal to x+1.317x which is 2.317x. The final expression is 2.317x is 81. The value of x is equal to 35 by calculation. The number of protons is 35. The number of protons is equal to the atomic number of any element. Thus, the symbolic representation of the element is $_{35}^{81}\text{Br}$ (mass number is 81 and atomic number is 35).
The correct answer of this question ‘Br’

So, the correct answer is “Option C”.

Additional Information:
Uses of Bromine:
- Bromine is a good oxidizing agent and it does not decompose easily.
- It liberates free iodine from iodide containing solutions and sulphur from hydrogen sulphide.
- Sulphurous acid is oxidized to sulphuric acid by bromine water.

Note: The mass number of the element is given 81 in the question. So, the element with mass number 81 is Bromine in the periodic table. The answer to this question can be obtained directly by just knowing the mass numbers of elements.