A Level Chemistry Revision "Ionisation Energy across a Period"

Introduction

This tutorial focuses on ionisation energy across Period 2 of the periodic table, a crucial concept in A Level Chemistry. Understanding ionisation energy helps explain various chemical properties and trends. This guide will break down the factors influencing ionisation energy and highlight key exceptions, making it easier to grasp the material for your studies.

Step 1: Understand Ionisation Energy

• Definition: Ionisation energy is the energy required to remove one mole of electrons from one mole of atoms in their gaseous state.
• Trend Across a Period: Generally, ionisation energy increases across a period due to:
  • Increased nuclear charge (more protons).
  • Electrons being added to the same energy level, leading to less shielding effect.
• Implication: Higher ionisation energy means it's harder to remove an electron, indicating stronger attraction between electrons and the nucleus.

Step 2: Examine Period 2 Elements

• Elements in Period 2: Lithium (Li), Beryllium (Be), Boron (B), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), Neon (Ne).
• Ionisation Energy Values: Note the increasing trend in ionisation energies from Li to Ne, with values generally rising due to the reasons outlined in Step 1.

Step 3: Identify Exceptions to the Trend

• Key Exceptions:
  • The drop in ionisation energy from Be to B.
    • Reason: Beryllium has a filled 2s orbital, while Boron starts to fill the 2p orbital. The 2p electron is higher in energy and experiences more shielding, making it easier to remove.
  • The drop from N to O.
    • Reason: Nitrogen has three unpaired electrons, while Oxygen has a pair in one of the 2p orbitals, leading to increased electron-electron repulsion which lowers the ionisation energy.

Step 4: Compare Period 2 with Period 3

• Similar Trends: The trend of increasing ionisation energy continues from Period 3 (Na to Ar) but with some differences due to the larger atomic radius and increased electron shielding.
• Identify Key Points:
  • Understand that while the general trend is similar, the specific values and exceptions may differ.
  • Look for similar exceptions in Period 3, especially between groups where electron pairing occurs.

Conclusion

Understanding ionisation energy across periods is critical for mastering A Level Chemistry concepts. Key takeaways include the general increase in ionisation energy across Period 2, the exceptions between Be and B as well as N and O, and the comparison between Period 2 and Period 3. For further study, review the ionisation energies of other periods and explore how these trends apply to chemical reactions and bonding.