Lecture on Ionization Energy and Electron Affinity

Ionization Energy

• Definition: Energy required to remove an electron from an atom.
• Example: Neutral lithium atom (Li)
  • Electron configuration: 1s² 2s¹
  • Contains 3 protons (positive charge) and 3 electrons.
  • Outer electron (valence electron) is in the 2s orbital, shielded by core electrons from full nuclear charge.
  • Removal of outer electron forms a positive lithium ion (Li⁺) with a positive charge because of 3 protons and 2 electrons left.
  • Ionization energy is positive (requires energy, measured in KJ/mol) because the outer electron is attracted to the nucleus.

Electron Affinity

• Definition: Energy change when an electron is added to a neutral atom.
• Example: Adding an electron to a neutral lithium atom
  • New electron configuration: 1s² 2s²
  • Total of 4 electrons, an added electron is shielded from the nucleus by core electrons.
  • Energy is released (negative value, -60 KJ/mol) as the added electron is attracted by the nucleus.

Comparison with Ionization Energy

• Ionization Energy
  • Takes energy to remove an electron; energy input is positive.
• Electron Affinity
  • Energy is released when an electron is added; energy value is negative.

Exceptions and Examples

• Neon

  • Electron configuration: 1s² 2s² 2p⁶
  • Adding an electron requires energy as effective nuclear charge is zero (all 10 electrons shield each other).
  • Neon has no affinity for an extra electron, hence electron affinity is often considered zero.

• Beryllium

  • Similar to neon; adding an extra electron requires energy because it must go to a higher energy 2p orbital.
  • Electron affinity is effectively zero (or positive).

• General Trend

  • Across a period (e.g., from Boron to Fluorine), electron affinity generally becomes more negative (more energy released).
  • Increase in effective nuclear charge as you move across a period enhances electron affinity.
  • Exceptions
    • Nitrogen: Due to electron repulsion in half-filled p orbitals, doesn’t follow the trend and has low affinity.

Conclusion

• Ionization energy shows a clearer trend than electron affinity.
• Electron affinity has more exceptions and is harder to generalize.
• Across a period, general electron affinity increases, but moving down a group shows inconsistencies with no clear trend.