Which statement best describes the atomic weight of an element?

• A. It is the mass of the most abundant isotope of the element.
• B. It is the weighted average mass of all naturally occurring isotopes of the element, measured in atomic mass units.
• C. It is always a whole number for every element.
• D. It is the sum of the number of protons and neutrons in the nucleus of the element.

Answer: (B)

Verified step by step guidance

1. Understand that atomic weight (also called atomic mass) represents the average mass of all naturally occurring isotopes of an element, taking into account their relative abundances.
2. Recall that isotopes of an element have the same number of protons but different numbers of neutrons, resulting in different masses.
3. Recognize that the atomic weight is not simply the mass of the most abundant isotope, nor is it always a whole number, because it is a weighted average.
4. Know that the sum of protons and neutrons in a single isotope is called the mass number, which is always a whole number, but atomic weight averages these values over all isotopes.
5. Therefore, the atomic weight is calculated using the formula: \(\text{Atomic Weight} = \sum (\text{fractional abundance of isotope} \times \text{mass of isotope})\), which explains why it is a weighted average.