Which statement best describes relative atomic mass (Ar)?

Which statement best describes relative atomic mass (Ar)?

• A. Ar is the mass of an atom in grams.
• B. Ar is the average mass of an atom relative to 1/12 of carbon-12.
• C. Ar is the number of protons in an atom.
• D. Ar is the total mass of all electrons.

Answer: (B)

Relative atomic mass is a way to compare how heavy an atom is using a standard reference. That standard is 1/12 of the mass of a carbon-12 atom. So Ar is the average mass of one atom, expressed in atomic mass units, relative to that carbon-12 baseline. Because elements often have several isotopes with different masses, Ar is a weighted average that reflects how common each isotope is. It’s not the mass in grams (that would be a real mass, not a relative one), not the number of protons (that’s the atomic number), and not the total mass of electrons. This definition captures why the stated description is the correct one.

Relative atomic mass is a way to compare how heavy an atom is using a standard reference. That standard is 1/12 of the mass of a carbon-12 atom. So Ar is the average mass of one atom, expressed in atomic mass units, relative to that carbon-12 baseline. Because elements often have several isotopes with different masses, Ar is a weighted average that reflects how common each isotope is. It’s not the mass in grams (that would be a real mass, not a relative one), not the number of protons (that’s the atomic number), and not the total mass of electrons. This definition captures why the stated description is the correct one.