Chemistry / Unit 2: Atomic Structure
2.4 Isotopes and Average Atomic Mass
Notes
Isotopes are atoms of the same element with different numbers of neutrons, so different mass numbers.
- Carbon-12 and carbon-14 both have 6 protons but 6 and 8 neutrons.
- Isotopes have nearly identical chemical properties.
- The average atomic mass on the periodic table is a weighted average of the isotopes: multiply each isotope's mass by its abundance (as a decimal) and add.
- Example: chlorine is 75.78% Cl-35 (34.97 amu) and 24.22% Cl-37 (36.97 amu). $0.7578(34.97) + 0.2422(36.97) = 26.50 + 8.954 = 35.45$ amu.
- The average is closest to the most abundant isotope.
Practice
Boron is 19.9% boron-10 (10.01 amu) and 80.1% boron-11 (11.01 amu). What is its average atomic mass?
$0.199(10.01) + 0.801(11.01) = 1.99 + 8.82 = 10.81$ amu. 10.51 is a simple average that ignores abundance.
Which pair are isotopes?
Isotopes have the same atomic number but different mass numbers.
Copper has two isotopes: Cu-63 (62.93 amu) and Cu-65 (64.93 amu). Its average atomic mass is 63.55 amu. Which statement is true?
63.55 is closer to 62.93 than to 64.93, so Cu-63 (about 69%) is more common.
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