Even though atoms are very tiny pieces of matter, they have mass. Their masses are so small, however, that chemists often use a unit other than grams to express them—the atomic mass unit.
The atomic mass unit: (abbreviated u, although amu is also used) is defined as [latex]\frac{1}{12}[/latex] of the mass of a C-12 atom:
1 u=[latex]\frac{1}{12}[/latex] the mass of C-12 atom
It is equal to 1.661 × 10−24 g.
Masses of other atoms are expressed with respect to the atomic mass unit. For example, the mass of an atom of H-1 is 1.008 u, the mass of an atom of O-16 is 15.995 u, and the mass of an atom of S-32 is 31.97 u. Note, however, that these masses are for particular isotopes of each element. Because most elements exist in nature as a mixture of isotopes, any sample of an element will actually be a mixture of atoms having slightly different masses (because neutrons have a significant effect on an atom’s mass). How, then, do we describe the mass of a given element? By calculating an average of an element’s atomic masses, weighted by the natural abundance of each isotope, we obtain a weighted average mass called the atomic mass (also commonly referred to as the atomic weight) of an element.
For example, boron exists as a mixture that is 19.9% B-10 and 80.1% B-11. The atomic mass of boron would be calculated as (0.199 × 10.0 u) + (0.801 × 11.0 u) = 10.8 u. Similar average atomic masses can be calculated for other elements. Carbon exists on Earth as about 99% C-12 and about 1% C-13, so the weighted average mass of carbon atoms is 12.01 u.
The table in Chapter 21 “Appendix: Periodic Table of the Elements” also lists the atomic masses of the elements.
Example 6
What is the average mass of a carbon atom in grams?
Skill-Building Exercise
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What is the average mass of a tin atom in grams? The atomic mass of tin is 118.71 u.
Concept Review Exercises
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Define atomic mass. Why is it considered a weighted average?
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What is an atomic mass unit?
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The atomic mass is an average of an element’s atomic masses, weighted by the natural abundance of each isotope of that element. It is a weighted average because different isotopes have different masses.
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An atomic mass unit is 1/12th of the mass of a C-12 atom.
Key Takeaway
- Atoms have a mass that is based largely on the number of protons and neutrons in their nucleus.
Exercises
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What is the atomic mass of zinc in atomic mass units?
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What is the atomic mass of barium in atomic mass units?
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What is the average mass of a single magnesium atom in grams?
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What is the average mass of a single calcium atom in grams?
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What is the mass of 1.00 × 1024 aluminum atoms in grams?
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What is the mass of 5.00 × 1023 carbon atoms in grams?
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Which has more mass—1 tungsten atom or 11 oxygen atoms?
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Which has more mass—1 magnesium atom or 6 helium atoms?