Chlorine occurs in nature in two isotopic forms of masses 35 u and 37

Chlorine occurs in nature in two isotopic forms of masses 35 u and 37 u in the ratio of 3 : 1 respectively. What is the average atomic mass of the Chlorine atom ?

[amp_mcq option1=”36·1 u” option2=”35·5 u” option3=”36·5 u” option4=”35·1 u” correct=”option2″]

This question was previously asked in

UPSC NDA-2 – 2021

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Chlorine exists as two isotopes: $^{35}$Cl with a mass of 35 u and $^{37}$Cl with a mass of 37 u. They are present in nature in a ratio of 3:1. This means that out of every 4 chlorine atoms, approximately 3 have a mass of 35 u and 1 has a mass of 37 u. The average atomic mass is calculated as a weighted average of the masses of the isotopes based on their relative abundance. The relative abundance of $^{35}$Cl is 3/4 or 0.75, and the relative abundance of $^{37}$Cl is 1/4 or 0.25.

Average atomic mass = ($\text{Mass of isotope 1} \times \text{Fractional abundance of isotope 1}$) + ($\text{Mass of isotope 2} \times \text{Fractional abundance of isotope 2}$)
Average atomic mass = $(35 \text{ u} \times 3/4) + (37 \text{ u} \times 1/4)$
Average atomic mass = $(105/4) \text{ u} + (37/4) \text{ u}$
Average atomic mass = $(105 + 37)/4 \text{ u}$
Average atomic mass = $142/4 \text{ u}$
Average atomic mass = $35.5 \text{ u}$.

Atomic mass unit (u) is a standard unit used for indicating atomic and molecular masses. Average atomic mass is the value listed for an element on the periodic table. It reflects the natural isotopic composition of the element.

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