Defining Atomic Mass - A Conceptual Dilemma?
The atomic mass of an element is defined as the average mass of a single atom of an element calculated through the abundance of its naturally occurring isotopes. However, elements like Carbon (C-12, C-13, C-14) have naturally occurring isotopes with a very small difference in mass. Considering this, how would you define atomic mass for such elements with varying abundance and slightly different masses?
1 Answer
📌 CONCEPT: The atomic mass of an element is a weighted average of the masses of its naturally occurring isotopes, taking into account their relative abundance in nature.
📐 RULE / FORMULA: The atomic mass (A) is calculated using the formula: A = (f1 × m1 + f2 × m2 +... + fn × mn) / (f1 + f2 +... + fn), where f represents the relative abundance and m represents the mass of each isotope.
💡 WORKED EXAMPLE: Consider Carbon (C), with isotopes C-12 (98.9% abundance) and C-13 (1.1% abundance). The atomic mass of Carbon is A = (0.989 × 12 + 0.011 × 13) / (0.989 + 0.011) = 12.01 u.
⚠️ COMMON MISTAKE: Students often assume that atomic mass is the mass of the most abundant isotope, which is incorrect as it's a weighted average of all isotopes.
12 Jul 26
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