Periodicity Lecture Notes

Introduction

• Lecturer: Chris Harris, from allerttutors.com
• Focus: AQA specification for periodicity

Elements in the Periodic Table

• Ordered by: Proton number (not mass number)
• Groups: Vertical columns
  • Elements in the same group have the same number of electrons in their outer shell
  • Elements in the same group exhibit similar properties
• Periods: Horizontal rows
  • Elements in the same period have the same number of electron shells
• Element Blocks:
  • S-block: Highest energy electron in S orbital
  • P-block: Highest energy electron in P orbital
  • D-block: Highest energy electron in D orbital
  • F-block: Highest energy electron in F orbital

Atomic Radius

• Across a Period: Decreases (e.g., Period 3)
  • Increased nuclear charge pulls electrons closer to the nucleus
  • Shielding remains relatively constant
• Down a Group: Increases
  • Additional electron shells are added
  • Increased shielding effect

Melting Points within a Period (Period 3)

• Metals (Na, Mg, Al):
  • Have metallic bonding
  • Melting points increase with increasing positive charge and more delocalized electrons
• Silicon (Si):
  • Giant covalent (macromolecular) structure
  • Very high melting point due to many strong covalent bonds
• Phosphorus (P4):
  • Simple molecular structure
  • Lower melting point, determined by Van der Waals forces
• Sulfur (S8):
  • Simple molecular but larger molecule than P4
  • Higher melting point due to larger Van der Waals forces
• Chlorine (Cl2) and Argon (Ar):
  • Simple molecular with even lower melting points
  • Argon is monatomic, very small Van der Waals forces

Ionization Energy

• Definition: Minimum energy required to remove one mole of electrons from one mole of atoms in gaseous state
• First Ionization Energy:
  • Key factors: shielding, atomic size, and nuclear charge
  • Example: Na -> Na+ + e- (+495.8 kJ/mol)
• Down a Group: Ionization energy decreases
  • Increased atomic radius and more shielding reduce energy required
• Across a Period: Ionization energy generally increases
  • Increased nuclear charge with stable shielding increases energy required
• Successive Ionizations: Removal of more than one electron; each successive ionization requires more energy
  • Example: Mg -> Mg2+ + 2e-
  • Significant jumps when removing electrons from inner shells closer to the nucleus

Special Cases in Ionization Energy Trends

• Decrease at Aluminium (Al): Evidence for subshells
  • Al's outer electron in 3p1 is higher energy and slightly further from nucleus than Mg's 3s
• Decrease at Sulfur (S): Evidence for electron repulsion
  • S's 3p has paired electrons causing repulsion, thus less energy needed to remove one

Conclusion

• Key concepts: shielding, nuclear charge, atomic radius
• Importance of keywords in answering exam questions
• Resource for further study: PowerPoint available for purchase [link provided in video description]