The fractional atomic mass of chlorine is due to which of the following?

The correct option is Presence of isotopes.

Step-by-Step Explanation:

1. Concept of Atomic Mass:
The atomic mass of an element represents the average mass of its naturally occurring atoms measured in atomic mass units (u).

2. What are Isotopes?
Isotopes are atoms of the same element that have the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers).

3. Chlorine Isotopes in Nature:
Naturally occurring chlorine exists as a mixture of two major isotopes:
- Chlorine-35 (${}_{17}{}^{35}\text{Cl}$) with an atomic mass of approximately 35 u, making up about 75% of natural chlorine.
- Chlorine-37 (${}_{17}{}^{37}\text{Cl}$) with an atomic mass of approximately 37 u, making up about 25% of natural chlorine.

4. Calculating Average Atomic Mass:
Because naturally occurring chlorine is a mixture of these isotopes, its atomic mass is calculated as a weighted average based on their relative natural abundances:

$\text{Average Atomic Mass}=\left(35\times \frac{75}{100}\right)+\left(37\times \frac{25}{100}\right)$

$\text{Average Atomic Mass}=26.25+9.25=35.5\text{u}$

5. Conclusion:
The fractional atomic mass of chlorine (approx. 35.5 u) is not due to partial protons or neutrons, nor experimental error, but rather due to the weighted average of its naturally occurring isotopes.