Algonquin College Health Program Assessment (AC-HPAT) Practice Exam

How is atomic mass best defined?

1. The sum of electrons and protons in an atom

2. The total mass of the chemical element in atomic mass units

3. The weighted average of an isotope's masses

4. The mass of the protons only in the nucleus

The correct answer is: The weighted average of an isotope's masses

Atomic mass is best defined as the weighted average of the masses of an element's isotopes, which takes into account both the mass of each isotope and its relative abundance in nature. This definition is crucial because elements typically exist as a mixture of different isotopes, each with a different mass due to variations in the number of neutrons in their nuclei. Using a weighted average enables chemists to calculate a more accurate representation of atomic mass that reflects the proportionate presence of each isotope in a sample. For example, if an element has isotopes with significantly different masses, the more prevalent isotope has a greater influence on the final atomic mass than the less common ones. Additionally, this average is measured in atomic mass units (amu), which helps in understanding the scale of these atomic particles in relation to one another. This conceptual understanding is vital for tasks such as stoichiometry in chemical equations and determining the isotopic composition in various applications. This distinction highlights that atomic mass is not simply a sum of protons and neutrons, nor does it exclusively consider the protons present. Therefore, while other definitions might touch on aspects of atomic structure, they do not encompass the complete concept of atomic mass as accurately as the correct answer does.