AQA Chemistry A-level: Atomic Structure

3.1.1: Atomic Structure

Fundamental Particles

• Evolution of Atomic Models

  • Plum Pudding Model: Atoms as a sphere of positive charge with embedded electrons.
  • Electron Shell Model:
    • Central nucleus with orbiting electron shells.
    • Nucleus contains protons and neutrons (positive charge).
    • Rutherford scattering experiment (1911).
    • In neutral atoms, electrons = protons.

• Particle Properties

  • Proton: Relative charge +1, relative mass 1.
  • Neutron: Relative charge 0, relative mass 1.
  • Electron: Relative charge -1, relative mass 1/1840.

• Electron Shells

  • Maximum number of electrons per shell: 2n².
  • Electrons fill one shell before moving to the next.

Mass Number and Isotopes

• Definitions

  • Mass Number (A): Sum of protons and neutrons.
  • Atomic Number (Z): Number of protons.
  • Relative Atomic Mass (Ar): Mean atom mass divided by 1/12 of carbon-12 atom mass.

• Isotopes

  • Same element, same atomic number, different neutron number.
  • React chemically the same; differ in physical properties.

• Examples

  • Hydrogen Isotopes: Hydrogen, Deuterium, Tritium.

• Ions

  • Formed by gain or loss of electrons, resulting in charge.

Mass Spectrometry

• Purpose: Identify isotopes and determine relative atomic mass.
• Time of Flight (TOF) Mass Spectrometry
  • Process Steps:
    1. Ionisation: Vapourised sample ionised to +1.
    2. Acceleration: Ions accelerated towards a detection plate.
    3. Ion Drift: Ions deflected by magnetic field.
    4. Detection: Ions hit detection plate, generate charge flow.
    5. Analysis: Produce spectra for isotope abundance.
  • Charge Effects: 2+ ions show smaller radius path, half m/z value.
• Chlorine Spectra: Characteristic patterns due to isotopic combinations.

Electron Configuration

• Electron Orbitals: s, p, d, f orbitals correspond to different Periodic Table blocks.
  • Orbital Capacities: s = 2, p = 6, d = 10.
  • Filling Order: Lowest energy orbitals filled first.
• Spin
  • Electrons pair with opposite spins within orbitals.
• Configuration Rules
  1. Lowest energy orbitals filled first.
  2. Same spin electrons fill orbitals before pairing.
  3. No orbital holds more than 2 electrons.
• Exceptions: Unpaired electrons may rearrange for stability.

Ionisation Energy

• Definition: Energy needed to remove one mole of electrons from gaseous atoms.
• Trends
  • Period Trend: Increases across a period (decreasing atomic radius).
  • Group Trend: Decreases down a group (increasing atomic radius and shielding).
• Graph Interpretation: Large energy jumps indicate new energy levels.
• Aluminium Exception: Lower energy due to electron repulsion.