Lecture on Isotopes and Atomic Mass

Key Concepts

• Isotopes: Variants of elements with different neutron numbers.
  • Fluorine: Only naturally occurring isotope is Fluorine-19 (100% abundance).
  • Lithium: Composed of 7.59% Lithium-6 and 92.41% Lithium-7.
  • Carbon: Consists of 98.93% Carbon-12, 1.07% Carbon-13, and a negligible amount of Carbon-14.

Atomic Weight and Mass

• Atomic Weight: The weighted average of the masses of isotopes based on their natural abundance.
  • Utilized to explain discrepancies between expected and actual atomic masses listed in periodic tables.

Calculation of Weighted Averages

• Formula:
  • % Isotope 1 x Mass of Isotope 1 + % Isotope 2 x Mass of Isotope 2 = Atomic Weight.
  • Example with Boron:
    • Boron-10: 19.78%, Mass = 10.0129 amu
    • Boron-11: 80.22%, Mass = 11.00931 amu
    • Weighted Average Calculation:
      • 0.1978 x 10.0129 + 0.8022 x 11.00931 = 10.812 amu

Units and Significant Figures (Sig Figs)

• Atomic Mass Units: Used to express masses of atoms for simplicity.
  • Proton ≈ 1 amu, Neutron similar, Electron negligible.
• Significant Figures: Must follow rules for operations (multiplication/division, addition/subtraction) when calculating.

Mass Spectrometry

• Purpose: Technique to identify molecules by separating them by mass and indicating their abundance.
  • Used heavily in organic chemistry.
  • Involves charged particles moving through a magnetic field.

Example with Silver

• Mass Spectrum Analysis:
  • Intensity plotted against mass (amu).
  • Example Peaks: Silver-107 and Silver-109.
  • Calculation of Atomic Weight:
    • Convert relative intensity to actual percent abundance.
    • Utilize weighted average formula to find atomic weight.

Solving for Percent Abundance

• Method:

  • Use largest isotope peak as reference (100%), calculate others proportionally.
  • Ensure total abundance equals 100%.
  • Example:
    • Silver-107: 100 / (100 + 92.9) = 51.84%
    • Silver-109: 1 - 0.5184 = 48.16%

• Weight Calculation:

  • Percent abundance x atomic mass for each isotope and sum.
  • Apply sig fig rules for addition/subtraction.
  • Example Result: 107.87 amu for Silver.

General Advice

• Significant Figures: Essential to track throughout calculations.
• Weighted Averages Video: Recommend watching for deeper understanding.
• Mass Spectrometry Applications: Significant in advanced chemistry fields.

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Notes

• Understanding isotopic composition is critical for interpreting atomic weights on the periodic table.
• Mass spectrometry provides powerful insights into molecular structure and composition.