A-Level Chemistry: Periodicity

Definition of Periodicity

• Periodicity: The study of repeating patterns or trends in physical or chemical properties as elements move around the periodic table.
• Properties linked to an element's position in the periodic table.
• Language of Position:
  • Groups: Columns (up and down)
  • Periods: Rows
  • Blocks: Regions of the periodic table corresponding to the subshell of outer electrons.
    • S block: Elements with outer electrons in the S subshell (Groups 1 & 2).
    • P block: Elements with outer electrons in a P subshell (right side of the table).
    • D block: Transition elements with outer electrons in the D subshell.

Key Properties in Period 3

Atomic Radius

• Definition: Distance from the nucleus to the outermost electrons.
• Trend: Decreases across period 3.
  • Electrons in same energy level (no change in shielding).
  • Increased nuclear charge pulls electrons closer.
• Exam Tips: Know pattern and explanation for 3-mark questions or sketch a decreasing pattern.

Electronegativity

• Definition: Ability of an atom to attract electron density in a covalent bond.
• Trend: Increases across period 3 (excludes noble gases).
  • Linked to decreasing atomic radius and increased nuclear charge.
• Trend in Groups: Increases up a group (smaller size, closer to nucleus).
• Exam Tips: Define, explain patterns, understand graphs.

Ionisation Energy

• Definition: Energy required to remove a mole of electrons from a mole of gaseous atoms.
• Trend: Generally increases across period 3.
  • Smaller atomic radius and larger nuclear charge.
• Graph Patterns: General increase with dips (Mg to Al, P to S).
  • Mg to Al Dip: p subshell electrons (higher energy, easier to remove).
  • P to S Dip: Electron repulsion in paired 3p orbitals.
• Exam Tips: Explain patterns, recognize graphs, understand periodic patterns.

Melting and Boiling Points

• States of Matter: Solids, Liquids, Gases.
• Trend for Period 3:
  • Metals (Na, Mg, Al): Increase in melting point due to stronger metallic bonding.
    • Higher charge density and smaller ions as you move right.
  • Silicon: Highest due to giant covalent structure.
  • Nonmetals (P, S, Cl, Ar): Influenced by van der Waals forces.
    • Larger molecules or more electrons have higher melting points.
• Exam Tips: Explain forces and types of bonding affecting melting points, justify comparisons between elements.

Skills and Exam Preparation

• Define key concepts: periodicity, electronegativity, ionisation energy.
• State block based on element's position.
• Work with and complete graphs for atomic radius, electronegativity, ionisation energy, melting point.
• Explain property patterns across period 3.
• Apply understanding to other periods not directly studied.