Ncert Exercises & Solutions

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

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,
Mass number of the element = 81
$∴$ (Number of protons + number of neutrons) = 81

$\Rightarrow x + 1.317 x = 81$
$2.317 x = 81$
$x = \frac{81}{2.317} = 34.95 = 35$

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 ${}_{35}^{81}{Br}$ .

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