Sample Notes: Atomic Structure

AS Level Chemistry – Detailed Notes

Chapter 1: Atomic Structure

1.1 Particles in the Atom and Atomic Radius

Structure of the Atom

• Atoms consist of:
  • Nucleus: Small, dense center containing protons and neutrons.
  • Electron shells: Surrounding space containing electrons.
• Most of the atom is empty space.
• Electrons orbit in shells (energy levels).

Subatomic Particles

Atomic and Mass Numbers

• Atomic (Proton) Number (Z): Number of protons in an atom. Determines element.
• Mass (Nucleon) Number (A): Total number of protons + neutrons.
• Number of Neutrons = A – Z

Mass and Charge Distribution

• Almost all mass is in the nucleus.
• The nucleus is positively charged due to protons.
• Electrons occupy large volume compared to nucleus → most of atom is empty.

Behavior of Beams in an Electric Field

• Protons: Deflected toward negative plate (positive charge, large mass).
• Electrons: Deflected toward positive plate (negative charge, very light).
• Neutrons: No deflection (no charge).

Determining Subatomic Particle Counts

• Atoms:
  Protons = Electrons = Atomic Number
  Neutrons = Mass Number – Atomic Number
• Ions:
  Electrons = Atomic Number ± Charge
  Example: Mg²⁺ has 12 protons and 10 electrons.

Trends in Atomic and Ionic Radius

• Across a Period:
  • Radius decreases.
  • Increased nuclear charge pulls electrons closer.
• Down a Group:
  • Radius increases.
  • More electron shells = larger atomic size.
• Ionic Radius:
  • Positive ions: Smaller than atoms (fewer electrons, stronger pull).
  • Negative ions: Larger than atoms (more electrons, repulsion increases size).

1.2 Isotopes

Definition of Isotope

• Atoms of the same element (same number of protons) with different numbers of neutrons.
• Same atomic number (Z), different mass number (A).

Notation

• Written as: ²⁰Ne or _20^40Ca
  • Top number (A): Mass number
  • Bottom number (Z): Atomic number

Chemical vs Physical Properties

• Chemical Properties: Same
  • Because chemical behavior depends on electrons and proton number (not neutrons).
• Physical Properties: Different
  • Due to differences in mass and density.

1.3 Electrons, Energy Levels and Atomic Orbitals

Key Terms

• Shells (n): Principal energy levels (n = 1, 2, 3…).
• Sub-shells: s, p, d (contain orbitals).
• Orbitals: Regions of space where up to 2 electrons are likely to be found.
• Ground State: Lowest energy configuration of electrons.

Orbital Structure

Order of Increasing Energy (up to 4p)

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p

Electronic Configuration Rules

• Fill orbitals from lowest to highest energy.
• Use either full configuration or shorthand using noble gas symbol.
• Example: Fe
  Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
  Shorthand: [Ar] 3d⁶ 4s²

Electrons in Boxes Notation

• Represent orbitals as boxes, arrows as electrons.
  ↑↓ in a box shows a filled orbital.

Orbital Shapes

• s orbital: Spherical shape.
• p orbitals: Dumbbell shape, arranged in x, y, z directions.

Free Radical

• A species with one or more unpaired electrons.

1.4 Ionisation Energy

Definition

• First Ionisation Energy (IE₁): Energy required to remove one mole of electrons from one mole of gaseous atoms.

  Example:
  Na(g) → Na⁺(g) + e⁻

Equations for Higher Ionisations

• Second ionisation:
  Na⁺(g) → Na²⁺(g) + e⁻
• Third ionisation:
  Mg²⁺(g) → Mg³⁺(g) + e⁻

Trends in Ionisation Energies

• Across a Period:
  • Increases: Greater nuclear charge, smaller atomic radius, same shielding.
• Down a Group:
  • Decreases: More shielding, larger radius, weaker attraction.

Successive Ionisation Energies

• Sharp increases indicate removal from a new shell.
• Useful to identify group number or number of electrons in outer shell.

Factors Affecting Ionisation Energy

• Nuclear Charge: More protons = higher IE.
• Atomic Radius: Greater distance = lower IE.
• Shielding: More inner shells = lower IE.
• Sub-shell Repulsion: Electrons in same orbital repel → slight reduction in IE.

Using Successive IE Data

• Large jumps in IE indicate new electron shells.
• Can be used to deduce electronic configuration and position in Periodic Table.