Lecture Notes: Section 7.2 on Covalent Bonds

Overview

• Discussion of covalent bond formation
• Introduction to electronegativity and polarity

Covalent Bonds

• Formed by shared pairs of electrons
• No metals involved, use prefixes to name compounds
• Seven diatomic molecules (BrINClHOF): Bromine, Iodine, Nitrogen, Chlorine, Hydrogen, Oxygen, Fluorine
  • Share pairs of electrons to form covalent bonds

Formation

• Unlike ionic bonds which involve electron exchange, covalent bonds share electrons
• Electrons move at the speed of light, making both atoms think they own the shared electrons
• Achieves octet or noble gas configuration

Examples

• Fluorine (F2): Single covalent bond represented by a dashed line
• Water (H2O): Lewis structure shows shared and unshared electron pairs
  • Hydrogen needs only 2 electrons
• Carbon Dioxide (CO2): Double bond with four shared electrons
• Nitrogen (N2): Triple bond with three pairs of shared electrons

Structural Representations

• Molecular Formula: Composition of molecules
• Structural Formula: Shows shared pairs with lines
• Lewis Formula: Displays shared and unshared electron pairs
  • Helpful for determining 3D shapes of molecules

Electronegativity and Polarity

• Electrons not always shared equally
• Fluorine: Highest electronegativity, attracts electrons strongly
• Unequal sharing results in electron-rich and electron-poor regions
• Symbolism:
  • Electron-rich: σ⁻ (somewhat negative)
  • Electron-poor: σ⁺ (somewhat positive)

Measuring Electronegativity

• Ability of an atom to attract electrons
• Pauling Scale: Ranges from 4.0 (Fluorine) to 0.7
  • Francium nearly 0, weak attraction
• Electronegativity increases as elements approach Fluorine
• Difference Calculation:
  • 0 = Nonpolar Covalent
  • 0-2 = Polar Covalent

  • 2 = Ionic (electrons transferred)

Classification of Bonds

• Ionic: Differences > 2
  • Example: Cesium (0.7) and Chlorine (3.0), difference 2.3
• Polar Covalent: Differences > 0
  • Example: Hydrogen (2.1) and Sulfur (2.5), difference 0.4
• Covalent: Difference 0
  • Example: Nitrogen bond (N-N)

Examples and Applications

• Polar Covalent Bond Formation:
  • Non-metals are usually considered
  • Example: Hydrogen and Oxygen
• Nonpolar Covalent Bonds:
  • Same elements bonding, e.g., Bromine with Bromine
• Furthest Non-metals: Likely to form polar covalent bonds

Conclusion

• Importance of understanding bond types and predicting molecular behavior
• Emphasis on prior knowledge of metals vs. non-metals and bond types
• Ends discussion on section 7.2