Question

An element with mass no. 81 contains 31.7% more neutron as compared to protons so actomic no is:

• A. 35
• B. 42
• C. 55
• D. 13

Answer 1

Answer: (A)

35

Let the number of protons in the element be $p$ and no. of neutrons $n$.

∴ Number of neutrons in the element

$n=p+31.7$ percent of $p$

$n=p+0.317p$

$n=1.317p$

According to the question,

Mass number of the element = 81

∴ $n+p=81$

$\to p+1.317p=81$

$2.317p=81$

$p=\frac{81}{2.317}$

$p\approx 35$

Hence, the number of protons in the element i.e.,$p$ 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.