Molecular Shape, Polarity, and Intermolecular Forces

Molecular Shapes and VSEPR Theory

• Determining Shape: Requires determining valence electrons, drawing Lewis structure, then counting electron groups around the central atom.
  • Electron groups include single, double, triple bonds (each counts as $1$) and lone pairs (each counts as $1$).
  • The shape is determined by the arrangement of atoms, neglecting invisible lone pairs.
• Common Shapes & Examples:
  • Bent: $H<em>2S$, $H</em>2O$ (central atom with $4$ electron groups, $2$ bonding, $2$ lone pairs).
  • Trigonal Pyramidal: $PF<em>3$, $NH</em>3$, $NCl_3$ (central atom with $4$ electron groups, $3$ bonding, $1$ lone pair).
  • Tetrahedral: $CCl_4$ (central atom with $4$ electron groups, $4$ bonding, $0$ lone pairs).

Naming Covalent Compounds

• Follows specific rules for nonmetal-nonmetal compounds.
• Use prefixes (di-, tri-, etc.) for the number of atoms.
• Second element ends with '-ide'.
• Example: $H_2S$ is Dihydrogen Sulfide.

Molecular Polarity

• Polar Molecule: Asymmetrical distribution of valence electron density, resulting in an overall dipole. Dipoles from individual bonds do not cancel due to molecular geometry.
• Nonpolar Molecule: Symmetrical distribution of valence electron density; individual bond dipoles cancel due to molecular geometry.
• Determining Polarity:
  1. Determine individual bond polarity (electronegativity difference).
     • Example: C-O bond is polar ($O$ electronegativity $3.5$, $C$ electronegativity $2.5$; difference $=1.0$).
     • Example: O-H bond is polar ($O$ electronegativity $3.5$, $H$ electronegativity $2.1$; difference $=1.4$).
     • Example: C-H bond is nonpolar.
     • Example: $Br-Br$ bond is nonpolar (atoms are identical).
  2. Determine the molecular shape.
  3. Assess if bond dipoles cancel out due to geometry.
     • Example ($CO_2$): Linear molecule, two polar C=O bonds with dipoles pointing in opposite directions, cancelling each other, so entire molecule is nonpolar.
     • Example ($H_2O$): Bent molecule, two polar O-H bonds, dipoles do not cancel, so molecule is polar.
     • Example ($CCl_4$): Tetrahedral geometry, four polar C-Cl bonds arranged symmetrically, dipoles cancel, so molecule is nonpolar.
     • Example ($CHCl_3$): Tetrahedral but with one C-H bond (nonpolar) and three C-Cl bonds (polar), dipoles do not cancel, so molecule is polar.
• Significance: Polar molecules attract other polar molecules; nonpolar molecules attract other nonpolar molecules. (