Atomic mass is defined as which of the following?

Atomic mass is defined as which of the following?

• A. The number of protons
• B. The total number of neutrons
• C. The total number of electrons
• D. The average mass of all the isotopes of an element

Answer: (D)

Atomic mass represents the weighted average mass of all naturally occurring forms of an element, known as isotopes, expressed in atomic mass units. Isotopes have the same number of protons but different numbers of neutrons, so each isotope has a slightly different mass. The atomic mass on the periodic table isn’t the number of protons, neutrons, or electrons; it’s the average mass of all isotopes, with each isotope’s mass weighted by how abundant it is. For example, chlorine mainly has two isotopes with masses around 35 and 37, and their differing abundances yield a final atomic mass of about 35.45 u. This explains why atomic mass is a decimal rather than an integer. The other options point to counts for a single isotope or to electron count, which don’t determine the overall mass.

Atomic mass represents the weighted average mass of all naturally occurring forms of an element, known as isotopes, expressed in atomic mass units. Isotopes have the same number of protons but different numbers of neutrons, so each isotope has a slightly different mass. The atomic mass on the periodic table isn’t the number of protons, neutrons, or electrons; it’s the average mass of all isotopes, with each isotope’s mass weighted by how abundant it is. For example, chlorine mainly has two isotopes with masses around 35 and 37, and their differing abundances yield a final atomic mass of about 35.45 u. This explains why atomic mass is a decimal rather than an integer. The other options point to counts for a single isotope or to electron count, which don’t determine the overall mass.