# DBT JRF 2020 — Question 22

Exam: DBT JRF 2020 · Section: A · Topic: Chemistry · Difficulty: Easy

## Question

Chlorine is assigned an atomic weight of 35.5. This is due to

## Options

- **A.** none of the given options
- **B.** presence of half a neutron
- **C.** presence of isotopes
- **D.** presence of half a proton

## Answer

Option C

## Explanation

Chlorine (atomic number 17) exists as two naturally occurring stable isotopes: $^{35}\text{Cl}$ (75.77% natural abundance, mass ≈ 35) and $^{37}\text{Cl}$ (24.23% natural abundance, mass ≈ 37). The standard atomic weight is the weighted average: $(35 \times 0.7577) + (37 \times 0.2423) = 26.52 + 8.97 = 35.49 \approx 35.5$ . This is NOT due to half-neutrons or half-protons (which do not exist) but is a statistical average across all naturally occurring chlorine atoms.

Source: https://mypyqbuddy.com/questions/chlorine-is-assigned-an-atomic-weight-of-35-6a05a8a02adafec4702931fc