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Which of the following best defines the atomic mass unit (amu)?
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2. Atoms & Elements
Atomic Mass
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Which numerical setup can be used to determine the atomic mass of bromine, given its two naturally occurring isotopes, Br-79 (abundance: 50.69%, mass: 78.918 amu) and Br-81 (abundance: 49.31%, mass: 80.916 amu)?
A
(0.5069 + 0.4931) × (78.918 + 80.916)
B
(0.5069 × 80.916) + (0.4931 × 78.918)
C
(0.5069 × 78.918) + (0.4931 × 80.916)
D
(78.918 + 80.916) / 2
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Understand that the atomic mass of an element with multiple isotopes is the weighted average of the masses of its isotopes, where the weights are their relative natural abundances.
Convert the percentage abundances of each isotope into decimal form by dividing by 100. For Br-79, 50.69% becomes 0.5069, and for Br-81, 49.31% becomes 0.4931.
Set up the weighted average formula for atomic mass: \(\text{Atomic Mass} = (\text{fractional abundance of isotope 1}) \times (\text{mass of isotope 1}) + (\text{fractional abundance of isotope 2}) \times (\text{mass of isotope 2})\).
Substitute the given values into the formula: \(\text{Atomic Mass} = (0.5069) \times (78.918) + (0.4931) \times (80.916)\).
This numerical setup correctly accounts for both the mass and the relative abundance of each isotope to find the average atomic mass of bromine.
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