Electronegativity

Introduction

• Electronegativity: The tendency of an atom within a molecule to attract electrons towards itself. It plays a vital role in predicting chemical reactions and identifying types of bonds.
• Relevance: Comprehending these patterns aids in accurately predicting the behaviour of elements and chemical reactions.

What is Electronegativity?

• Electronegativity: The tendency of an atom to attract a shared pair of electrons.
• Importance: This concept is crucial for grasping chemical bonding and the properties of compounds.
• Dimensionless Quantity:
  • It is defined without units and is usually measured on the Pauling scale.

Periodic Trends in Electronegativity

• General Trends:
  • Increases across periods from left to right, due to the higher nuclear charge.
  • Decreases down groups as additional electron shells provide shielding, reducing the nuclear attraction.

Historical Development

• Key Contributor: Linus Pauling made notable contributions to the understanding of electronegativity.
• Progression: The concept evolved from qualitative ideas to quantitative scales.

Different Electronegativity Scales

• Scales Overview:
  • Pauling Scale: Most widely used and recognised.
  • Mulliken Scale: Considers both electron affinity and ionisation energy.
  • Allred-Rochow Scale: Based on electrostatic forces between nuclei and electrons.

Visual Representation

Bond Classification by Electronegativity

• Electronegativity Differences and Bond Types:
  • Ionic Bonds: Electronegativity difference greater than 1.7.
    • Example: Sodium chloride (NaCl).
  • Polar Covalent Bonds: Electronegativity differences between 0.5 and 1.7.
    • Example: Water (H₂O).
  • Non-Polar Covalent Bonds: Electronegativity difference less than 0.5.
    • Example: Chlorine gas (Cl₂).

Metal Reactivity and Electronegativity

• Inverse Relationship: Metals with low electronegativity are typically highly reactive, especially within the alkali metal group.
• Reactive Metals:
  • Lithium (0.98), Sodium (0.93), Potassium (0.82) are notably reactive.

Common Misconceptions

• Noble Gases: Typically excluded from electronegativity considerations due to their full valence electron shells.
• Visualising Trends: Use diagrams to clarify misunderstandings.

Exam Tips

• Understanding Trends: Remember that electronegativity increases across a period and decreases down a group.
• Utilise Visuals: Charts and diagrams are invaluable study aids.
• Practice Problems: Apply scenarios to comprehend how electronegativity influences bonding.

Example Questions

1. Determine the type of bond that forms between lithium (Li) and fluorine (F). Solution: Electronegativity of Li = 0.98, F = 3.98. Difference = 3.98 - 0.98 = 3.0, indicating an ionic bond.

2. Predict the bond type in carbon dioxide (CO₂). Solution: Electronegativity of C = 2.55, O = 3.44. Difference = 3.44 - 2.55 = 0.89, indicating a polar covalent bond.

Worked Examples

• NaCl: Calculate $\Delta EN = |3.16 - 0.93| = 2.23$, indicating an ionic bond.
• H₂O: Calculate $\Delta EN = |3.44 - 2.20| = 1.24$, indicating a polar covalent bond.