Covalent Bonds and Molecular Structures: Key Concepts

Posted on Jan 30, 2025 in Chemistry

Covalent Bonds and Molecular Structures

Types of Covalent Bonds

• Non-polar Covalent Bonds: Electronegativity difference is close to zero (less than 0.4).
• London Dispersion Force: Weak attraction between nonpolar molecules; the more electrons, the greater the attraction.
• Polar Covalent Bonds: Electronegativity difference is between 0.4 and 1.67. The higher the difference, the more polar the bond.

Dipole Moment

A dipole moment (polar molecule) is a molecule that has a center of positive charge and a center of negative charge. It must be a covalent bond.

Example: A water molecule has hydrogen “intermolecular attraction” bonding.

Nitrogen, oxygen, and fluorine undergo hydrogen bonding due to their high electronegativity.

Polar vs. Non-polar Molecules

• Polar Molecule: Net dipole as a result of polar bonds arranged asymmetrically.
• Non-polar Molecule: Equal sharing of electrons in a diatomic molecule or symmetrical arrangement of polar bonds.

Resonance Structures

Resonance occurs when more than one Lewis structure can be drawn for a molecule.

Polyatomic Ions

A polyatomic ion (also known as a molecular ion) is an ion composed of two or more atoms covalently bonded.

Coordinate Covalent Bond

A coordinate covalent bond is a covalent bond in which both electrons come from the same atom.

Examples:

• Ammonia (NH₃) to ammonium ion (NH₄⁺)
• Water (H₂O) to hydronium ion (H₃O⁺)

Characteristics of Covalent Compounds

• Low melting points
• Non-conductive
• Flammable
• Soft
• Less soluble
• London dispersion forces

Exception: Network solids (e.g., diamonds).

VSEPR Theory and Hybridization

VSEPR (Valence Shell Electron Pair Repulsion) theory states that electron pairs around a central atom will arrange themselves to minimize repulsion.

Hybridization occurs when two orbitals combine to minimize repulsion between electron pairs.

Example: Mixing an “s” and a “p” orbital produces two “sp” hybrid orbitals.

Common Hybridization and Molecular Geometries

• SP: AX₂ – Linear
• SP²:
  • AX₃ – Trigonal Planar
  • AX₂E – Bent
• SP³:
  • AX₄ – Tetrahedral
  • AX₃E – Trigonal Pyramidal
  • AX₂E₂ – Bent
• SP³D:
  • AX₅ – Trigonal Bipyramidal
  • AX₄E – Seesaw
  • AX₃E₂ – T-Shaped
• SP³D²:
  • AX₆ – Octahedral
  • AX₅E – Square Pyramidal
  • AX₄E₂ – Square Planar

SNAP Rule

Symmetrical = Nonpolar molecule
Asymmetrical = Polar molecule

Electronegativity and Bond Polarity

• Polar covalent bond: Electronegativity difference between 0.4 and 1.67.
• Nonpolar covalent bond: Electronegativity difference of 0.4 or less.

Practice Questions

1. As the difference in electronegativity between two atoms decreases, the tendency for the formation of covalent bonds:
   • Always increases
2. In a nonpolar covalent bond, electrons are:
   • Shared equally by two atoms
3. The atoms in a molecule of hydrogen chloride are held together by:
   • Polar covalent bonds
4. A proton (H⁺) would most likely form a coordinate covalent bond with:
   • H₂O
5. Diamond and graphite both have bonds that are predominantly:
   • Network
6. Which is a polar molecule?
   • HCl
7. Hydrogen bonds are formed between molecules in which hydrogen is covalently bonded to an element having:
   • High electronegativity
8. Which molecule is nonpolar?
   • BF₃
9. A pure substance melts at 113°C and does not conduct electricity in either the solid or liquid state. The bonding in this substance is primarily:
   • Covalent molecular
10. Multiple covalent bonds exist in a molecule of:
    • N₂
11. Which type of bonding is usually exhibited when the electronegativity difference between two atoms is 1.1?
    • Covalent
12. Which molecule can form a coordinate covalent bond?
    • NH₃
13. Which is a dipole molecule?
    • BrCl
14. At 298 K, the vapor pressure of H₂O is less than the vapor pressure of CS₂. The best explanation for this is that H₂O has:
    • Stronger intermolecular forces
15. Which will not conduct an electric current in either the molten or solid state?
    • SiF₄