Periodicity

• What is the definition of periodicity in chemistry?

  Trends in properties across the periodic table
• What properties are associated with periodicity?

  Atomic radius, melting point, boiling point, ionisation energy
• How does atomic radius change across a period?

  It decreases across a period
• Why does atomic radius decrease across a period?

  Increased positive charge pulls electrons closer
• What effect do extra electrons have on atomic radius across a period?

  They do not provide extra shielding effect
• What is the atomic radius of sodium (Na)?

  0.186 nm
• What is the atomic radius of chlorine (Cl)?

  0.100 nm
• How do melting points change across Period 3?

  They increase then decrease from Na to Ar
• Why do melting points increase from sodium to silicon?

  Stronger metal-metal bonds due to increased charge
• What is the structure of silicon?

  Macromolecular with a tetrahedral structure
• What type of substances are phosphorus, sulfur, and chlorine?

  Molecular substances
• What determines the melting points of phosphorus, sulfur, and chlorine?

  Strength of van der Waals forces between molecules
• Why does sulfur have a higher melting point than phosphorus or chlorine?

  It has a larger molecule (S₈)
• What is the melting point of argon?

  Very low due to monatomic structure
• What is the first ionisation energy?

  Energy to remove 1 electron from 1 mole of atoms
• How does first ionisation energy change across Period 3?

  It generally increases across the period
• Why does first ionisation energy increase across Period 3?

  Increased attraction between outer electrons and nucleus
• What are the trends in atomic radius, melting points, and ionisation energy across Period 3?

  • Atomic radius decreases across the period
  • Melting points increase from Na to Si, then decrease to Ar
  • First ionisation energy increases across the period
• What are the bonding and structural differences between metals and non-metals in Period 3?

  • Metals (Na, Mg, Al):
  • Metallic bonding
  • Stronger melting points due to metal-metal bonds
  • Non-metals (Si, P, S, Cl, Ar):
  • Covalent or van der Waals forces
  • Lower melting points due to weaker forces