2.6: Atomic Masses - Chemistry LibreTexts

Learning Objectives

• Define atomic mass and atomic mass unit (amu).
• Calculate atomic mass.

Atomic Mass Unit (amu)

• Definition: 1 amu is defined as 1/12 of the mass of a 12C atom.
  • 1 u = 1.661 x 10^-24 g.
• Masses of atoms are expressed with respect to the amu.
  • Examples:
    • 1H = 1.008 u
    • 16O = 15.995 u
    • 32S = 31.97 u
• Note: These are masses for particular isotopes.

Atomic Mass as Weighted Average Mass of Isotopes

• Most elements exist as mixtures of isotopes.
• Weighted Average Mass (Atomic Mass/Atomic Weight):
  • Calculate as the average of atomic masses, weighted by the natural abundance of each isotope.
• Examples:
  • Hydrogen
    • Isotopes: H-1 (99.985%, 1.0078 amu), H-2 (0.015%, 2.0141 amu), H-3 (negligible, 3.0160 amu)
    • Average Atomic Mass: 1.0079 amu
  • Chlorine
    • Isotopes: Cl-35 (75.77%, 34.969 amu), Cl-37 (24.23%, 36.966 amu)
    • Average Atomic Mass: 35.453 amu
    • Formula: 0.7577(34.969u) + 0.2423(36.966u) = 35.453u

Calculation Examples

• Oxygen:

  • Isotopes: 16O (99.759%, 15.995 u), 17O (0.037%, 16.995 u), 18O (0.204%, 17.999 u)
  • Atomic Mass Calculation: 0.99759(15.995u) + 0.00037(16.995u) + 0.00204(17.999u) = 15.999u

• Copper:

  • Isotopes: 63Cu (69.17%, 62.930 u), 65Cu (30.83%, 64.928 u)
  • Atomic Mass: 63.546 u

Key Takeaways

• Atoms have mass largely determined by the number of protons and neutrons.
• Atomic mass in the Periodic Table is the weighted average of the mass of all isotopes.

Concept Review Exercises

1. Define atomic mass: An average of atomic masses, weighted by the isotope's natural abundance.
2. Define atomic mass unit: 1/12 of the mass of a 12C atom.

Note: The atomic mass of elements such as tin and carbon can be found in the Periodic Table under their symbols.