Atomic Mass: A Misconception?
Atomic masses are averages of naturally occurring isotopes of an element. What might lead to a difference in atomic mass of an element from its isotopic masses? Illustrate with an example of an element with two naturally occurring isotopes and explain why its atomic mass is likely to be different from the sum of its isotopic masses.
1 Answer
📌 CONCEPT: The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes.
📐 RULE / FORMULA: The atomic mass is calculated using the formula: atomic mass = (abundance of isotope 1 × mass of isotope 1) + (abundance of isotope 2 × mass of isotope 2) + ... .
💡 WORKED EXAMPLE: Consider an element, carbon, which has two naturally occurring isotopes: carbon-12 (99.9% abundance, mass = 12 u) and carbon-13 (0.1% abundance, mass = 13 u). Using the formula, the atomic mass of carbon would be (0.999 × 12) + (0.001 × 13) = 12.002 u. The sum of the isotopic masses is 12 + 13 = 25 u, which is different from the atomic mass of carbon.
⚠️ COMMON MISTAKE: Students often assume that the atomic mass of an element is the sum of the masses of its isotopes, which is incorrect due to the varying abundance of isotopes.
10 Oct 26
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