Lecture on Atomic Mass

Overview

• Atomic mass is a fundamental characteristic of elements.
• Often a weighted average, not a simple one.
• Found on the periodic table under the element’s symbol.

Key Concepts

Atomic Mass

• Defined as an average mass of an element's atoms.
• Calculated as a weighted average, considering the existence of isotopes.

Isotopes

• Variants of an element with the same number of protons but different numbers of neutrons.
• Example: Copper has two main isotopes, Copper-63 and Copper-65.

Weighted Averages

• Different from regular averages learned in math.
• Takes into account various quantities of each subtype (e.g., isotopes).

Weighted Average Calculation

Analogy with Cars

• Lemona GX: 4,000 pounds
• Lemona GXL: 5,000 pounds
• 95% are GX, 5% are GXL.

Regular Average

• Add weights: 4000 + 5000 = 9000
• Divide by 2 = 4500 pounds

Weighted Average Formula

• GX contribution: 4000 pounds * 0.95
• GXL contribution: 5000 pounds * 0.05
• Resulting weighted average = 4050 pounds

Application to Atomic Mass

• Copper-63 mass: 63 amu, abundance: 69%
• Copper-65 mass: 65 amu, abundance: 31%

Calculation

• Copper-63 contribution: 63 amu * 0.69
• Copper-65 contribution: 65 amu * 0.31
• Resulting weighted average = 63.62 amu

Additional Details

• Real atomic masses and abundances have more precise values than the simplified numbers used here.

Conclusion

• Atomic mass involves understanding and calculating weighted averages of isotopes.
• Practice problems available in other materials.