Isotopes and Mass Spectrometry

Jul 3, 2024

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Isotopes and Mass Spectrometry

IES Chemistry Course Overview

  • Topic focus: Structure of the atom, isotopes, and mass spectrometry
  • Review of subatomic particles: protons, neutrons, electrons
  • Atomic number and mass number definitions
  • Determining subatomic particles using atomic and mass numbers
  • Concept of isotopes
  • Introduction to mass spectrometry principles and mass spectra for diatomic molecules

Structure of the Atom

  • Subatomic particles:
    • Protons
    • Neutrons
    • Electrons
  • Mass and charge:
    • Proton: Mass = 1, Charge = +1
    • Neutron: Mass = 1, Charge = 0 (neutral)
    • Electron: Mass = 1/1840 or 1/1836, Charge = -1
  • Location:
    • Protons and neutrons: Nucleus
    • Electrons: Quantum shells (energy levels)

Atomic Number and Mass Number

  • Atomic number: Number of protons in an atom
  • Mass number: Number of protons + neutrons
  • Using atomic and mass numbers to determine subatomic particles

Isotopes

  • Atoms of the same element with the same atomic number but different mass numbers
  • Examples: Chlorine-35 and Chlorine-37
    • Chlorine-35: 18 neutrons
    • Chlorine-37: 20 neutrons
  • Definition used in explanations may involve subatomic particles or atomic/mass number

Relative Atomic Mass

  • Definition: Weighted mean average mass of an atom of an element compared to 1/12 of the mass of carbon-12
  • Calculation involves relative isotopic masses and percentage abundances
  • Example calculation for Lithium:
    • Isotopic masses: 6.015 and 7.016
    • Percentage abundances: 7.59% and 92.41%
    • Formula: (Mass * Abundance) + (Mass * Abundance) / 100

Mass Spectrometry

  • First mass spectrometer: 1918 by Francis Aston
  • Used to prove the existence of isotopes
  • Modern use: Calculate molecular masses and analyze new compounds

Mass Spectrometer Process

  1. Vaporization: Convert sample to gas
  2. Ionization: Convert particles to positive ions (typically 1+ charge)
  3. Acceleration: Using electric field
  4. Deflection: Using magnetic field, depends on mass-to-charge ratio (m/z)
  5. Detection: Detect mass-to-charge ratios to identify isotopes
  • Path depends on m/z ratio:
    • Larger m/z value: Less deflection
    • Higher charge (2+): More deflection

Interpreting Mass Spectra

  • Peak positions: Indicate the mass of substances
  • Peak intensity/height: Represents abundance
  • Example: Mass spectra for Chlorine (diatomic molecule)
    • Peaks at 35, 37, 70, 72, 74
    • Calculation of peak heights using probability based on abundance

Advanced Use

  • Analyzing diatomic molecules
    • Formation of peaks due to different isotopes combining
    • Ratio and percentage abundance calculations
  • Practice drawing mass spectra and solving related questions for exams

Example Questions

  • Identifying isotopic masses
  • Determining particles responsible for mass spectrometry peaks
  • Explaining peak heights based on relative abundance probabilities
  • Calculating relative atomic and molecular masses
  • Impacts of ion charge variations on m/z values

Practice Recommendations

  • Practice drawing mass spectra and solving textbook questions
  • Use exam papers to familiarize with question formats and marking schemes

For more questions, leave a comment below.

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