Understanding Atomic Structure and Mass Spectrometry

Sep 21, 2024

A Level Chemistry: Atomic Structure

Introduction

  • Chemical properties depend on atomic structure, especially electron arrangement around the nucleus.
  • Focus of the lecture includes:
    • Fundamental particles
    • Mass number and isotopes
    • Electron configuration
    • Ionization
    • Mass spectrometer and substance analysis

Fundamental Particles

  • Understanding of atomic models has evolved:
    • Billiard Ball Model: Atoms are solid lumps.
    • Plum Pudding Model: Atoms contain electrons diffused in a positive mass.
    • Nuclear Model: Atoms have a small nucleus with electrons orbiting.
    • Bohr Model: Electrons are arranged in specific energy levels.

Key Points on Particles

  • Nucleus contains protons (+1 charge) and neutrons (0 charge), held together by the strong nuclear force.
  • Relative mass:
    • Protons/Neutrons = 1
    • Electrons = 1/1840 of a proton.
  • Atomic number (Z) = number of protons; Mass number (A) = protons + neutrons.

Isotopes

  • Isotopes have the same number of protons but different neutrons.
  • Reactivity is determined by electron configuration rather than isotopes' mass.

Example Calculations

  • Fluorine (Z=9, A=19):
    • Protons = 9, Electrons = 9, Neutrons = 10.
  • Sodium ion (Na+):
    • Protons = 11, Electrons = 10, Neutrons = 12.

Electron Configuration

  • Electrons arranged in shells/energy levels.
    • Carbon: 6 electrons (2 in first shell, 4 in second) = 2, 4.
    • Lithium: 3 electrons (2 in first, 1 in second) = 2, 1.

Energy Levels and Sublevels

  • Shells can hold:
    • 1st Shell: 2 electrons
    • 2nd Shell: 8 electrons
    • 3rd Shell: 18 electrons
  • Sublevels: S, P, D, F, each with specific shapes and energy levels.

Filling Rules

  1. Fill lower energy levels first.
  2. Fill orbitals singly before pairing.
  3. Max of 2 electrons per orbital.

Examples of Electron Configuration

  • Carbon: 1sĀ² 2sĀ² 2pĀ²
  • Oxygen: 1sĀ² 2sĀ² 2pā“
  • Iron: 1sĀ² 2sĀ² 2pā¶ 3sĀ² 3pā¶ 4sĀ² 3dā¶
  • Transition metals: 4s fills before 3d.

Ionization Energy

  • Energy required to remove one mole of electrons from a mole of gaseous atoms.
  • Increases with removal of electrons due to increased positive charge on ion.
  • Trends:
    • Across periods: Generally increases.
    • Down groups: Generally decreases due to increased atomic radius and shielding effect.

Mass Spectrometry

  • Measures mass of atoms/molecules and identifies elements.
  • Steps in Time-of-Flight Mass Spectrometer:
    1. Ionization: Creates positive ions.
    2. Acceleration: Ions gain kinetic energy.
    3. Drift: Lighter ions travel faster.
    4. Detection: Ions hit detector, generating current; abundance determined.

Types of Ionization

  1. Electron Gun: Electrons knock off to create ions.
  2. Electrospray Ionization: Dissolved sample gains a proton.

Interpreting Mass Spectra

  • Peaks represent isotopes and molecular ion peaks indicate molecular mass.
  • Fragmentation occurs when molecules break into smaller pieces.

Average Atomic Mass Calculation

  • Average mass calculated using isotope abundances.

  • Formula:

    [ \text{Average Mass} = \frac{(abundance_1 \times mass_1) + (abundance_2 \times mass_2)}{total\ abundance} ]

Example of Average Mass

  • Chlorine with 75% Cl-35 and 25% Cl-37:
    [ \text{Average} = \frac{(75 \times 35) + (25 \times 37)}{100} = 35.5 ]

Summary

  • Understanding atomic structure is key for explaining chemical properties.
  • Ionization energy trends relate to atomic structure and electron configuration.
  • Mass spectrometry provides vital data on isotopes and molecular masses.