The relative atomic mass of chlorine is not a whole number because

Analyze the question: It asks why chlorine's relative atomic mass (about 35.5) is not a whole number.

Concept: Most elements exist as a mixture of isotopes. These are atoms with the same number of protons but different numbers of neutrons, so they have different mass numbers. The relative atomic mass is the weighted average of the masses of these isotopes, with each isotope's mass weighted by its natural abundance.

Solution for chlorine: Chlorine has two main isotopes:

• Chlorine-35, about 75% abundance
• Chlorine-37, about 25% abundance

Relative atomic mass = (35 × 75/100) + (37 × 25/100) = 26.25 + 9.25 = 35.5

Each isotope has a whole-number mass, but the average is not a whole number because the isotopes occur in different proportions.

Checking the options:

• A. Correct. It states that the value is the weighted average mass of all its isotopes, which is exactly how 35.5 arises.
• B. This is close, but "average mass" without "weighted" is incomplete. A simple average of 35 and 37 would be 36, not 35.5. The abundances are what matter.
• C. Oxidizing ability has nothing to do with atomic mass.
• D. Having more neutrons than protons does not make the mass fractional. Mass numbers of single atoms are whole numbers.

Answer: A

Exam tip: When you see "not a whole number" in a relative atomic mass question, think isotopes + relative abundance = weighted average. Watch for the word "weighted", because it often separates the best answer from a near-miss option.