Periodic Trends and Coulomb's Law

Introduction

• Subject: Understanding periodic trends in the periodic table using Coulomb's Law.
• Key Concepts: Ionization energy, atomic and ionic radii, electron affinity, and electronegativity.

Coulomb's Law

• Defines the magnitude of force between two charged particles.
• Formula: Force is proportional to the product of charges divided by the square of the distance between them.
• Application to Atoms:
  • q1: Effective positive charge from protons in the nucleus.
  • q2: Charge of an electron (constant negative charge).
  • Distance considered is between the nucleus and the valence electrons.

Effective Nuclear Charge (z-effective)

• Definition: Difference between nuclear charge and shielding effect from core electrons.
• Approximation: Charge of the nucleus minus the number of core electrons.
• Group I Elements: Effective charge is approximately 1.
• Halogens: Effective charge is higher, approximately 7.
• Noble Gases: Effective charge of 8, except Helium which is 2.

Trends in the Periodic Table

• Left to Right:
  • Effective charge increases.
  • Atomic radius decreases due to stronger attractive force as per Coulomb's Law.
• Within a Column:
  • Radius increases as you add more electron shells.

Ionization Energy

• Definition: Energy required to remove an electron from an atom.
• Trend:
  • Highest at the top right of the periodic table.
  • Noble gases have high ionization energy due to high stability.
  • Francium has low ionization energy.

Electron Affinity

• Definition: Energy change when an electron is added to a neutral atom.
• Trend:
  • High electron affinity at the top right, especially halogens.
  • Conventionally described as negative when energy is released.

Electronegativity

• Definition: Tendency of an atom to attract a bonding pair of electrons.
• Correlation: Strongly correlates with electron affinity.

Conclusion

• Coulomb's Law helps intuitively understand trends in the periodic table.
• Effective charge and atomic radius are key factors in determining periodic trends.