What term describes the average mass of an element's isotopes?

What term describes the average mass of an element's isotopes?

• A. Atomic number
• B. Atomic weight
• C. Mass number
• D. Valence

Answer: (B)

The concept being tested is how we describe the average mass of an element when its atoms come in different isotopes. Because isotopes have different masses and occur in different natural abundances, the overall or average mass is found by taking a weighted average of those isotopic masses. This weighted average is the atomic weight, the value you see on the periodic table for each element. It reflects how common each isotope is in nature, not the mass of a single atom. In contrast, atomic number is the number of protons in an atom, which defines the element; mass number refers to the total count of protons and neutrons in a specific isotope; and valence describes the bonding behavior of an atom’s outer electrons. So the term that best describes the average mass of an element’s isotopes is atomic weight.

The concept being tested is how we describe the average mass of an element when its atoms come in different isotopes. Because isotopes have different masses and occur in different natural abundances, the overall or average mass is found by taking a weighted average of those isotopic masses. This weighted average is the atomic weight, the value you see on the periodic table for each element. It reflects how common each isotope is in nature, not the mass of a single atom.

In contrast, atomic number is the number of protons in an atom, which defines the element; mass number refers to the total count of protons and neutrons in a specific isotope; and valence describes the bonding behavior of an atom’s outer electrons. So the term that best describes the average mass of an element’s isotopes is atomic weight.