Gen chem

• Molecular geometry


  The arrangement of atoms in a molecule
• Electron-domain geometry


  The arrangement of electron domains (bonding pairs and lone pairs) around the central atom
• Determining molecular geometry using VSEPR model

  1. Determine the number of electron domains around the central atom

  2. Arrange the electron domains to minimize repulsion (linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral)

  3. Identify the molecular geometry based on the arrangement of atoms (excluding lone pairs)
• The molecular geometry of a molecule is determined by the arrangement of atoms, excluding lone pairs
• The electron-domain geometry is determined by the arrangement of all electron domains (bonding pairs and lone pairs) around the central atom
• The electron-domain geometry


  Determines the molecular geometry
• The VSEPR model predicts the electron-domain geometry based on minimizing repulsion between electron domains
• VSEPR model


  Used to predict the electron-domain geometry
• Predicting molecular geometry


  1. Draw Lewis structure

  2. Determine electron-domain geometry

  3. Describe molecular geometry
• Steps to predict molecular shapes using VSEPR model

  • Draw Lewis structure and count total number of electron domains
  • Determine electron-domain geometry to minimize repulsions
  • Use arrangement of bonded atoms to determine molecular geometry
• Molecules with expanded valence shells

  • Central atom from 3rd period or beyond may have more than 4 electron pairs
  • Trigonal bipyramidal (5 electron domains)
  • Octahedral (6 electron domains)
• Nonbonding electron domains occupy equatorial positions in trigonal bipyramidal geometry
• Trigonal bipyramidal geometry


  Two trigonal pyramids sharing a base<|>Two axial positions<|>Three equatorial positions
• Octahedral geometry


  Polyhedron with 8 faces and 6 vertices<|>All bond angles are 90°<|>All vertices are equivalent
• VSEPR model can be extended to more complex molecules
• Acetic acid molecule has three interior atoms with different geometries
• Bond angles in acetic acid deviate slightly from ideal values due to spatial demands
• Molecules to classify by molecular geometry

  • H2O
  • CH4
  • SF4
  • H2S
  • OF2
  • CO2
  • XeF4
  • BrF5
  • I3-
  • CO3-2
  • NH3
  • PCl5
  • SF6
  • O3
  • HCN
  • NO2-
  • XeF2
  • ClF3
  • SiCl4
  • ICl3
• Approximate bond angles in given molecules
• Polar covalent bond


  When there is a difference between the electronegativity of the two elements, such as between two different nonmetals, the bond is polar covalent
• Polar covalent bond


  • HF
• Nonpolar (pure) covalent bond

  When the two atoms sharing the electrons have the same electronegativity, the bond is nonpolar or pure covalent
• Nonpolar covalent bond


  • Cl2
• Ionic bond


  When there is a large difference in electronegativity between the two atoms, one atom completely transfers an electron to the other, forming an ionic bond
• Ionic bond


  • NaCl
• Effect of electronegativity difference on bond type

  • Zero (0-0.4): Pure/Nonpolar covalent
  • Intermediate (0.5-1.7): Polar covalent
  • Large (>1.7): Mostly ionic
• Determining bond type


  1. Find electronegativity difference

  2. Classify bond as nonpolar covalent, polar covalent, or ionic
• Polar molecule
• Nonpolar molecule
• Symmetric shapes


  • Linear, trigonal planar, tetrahedral
• Unsymmetric shapes


  • Bent, trigonal pyramidal
• Determining molecular polarity


  1. Draw Lewis structure

  2. Determine molecular shape

  3. If shape is unsymmetric, molecule is polar

  4. If shape is symmetric, check if all outside atoms are the same - if not, molecule is polar
• Polar and nonpolar molecules behave differently
• Molecules to classify


  • PCl3
  • CCl4
  • BH3
  • CH3Cl
  • SO2
  • SO3
  • H2O
  • NH3
  • HCN
  • CH4
  • CH2O
  • O3
  • CH3CH3
  • OF2
  • N2
• Polar bond


  Bond where electrons are not evenly shared between atoms
• Nonpolar bond


  Bond where electrons are evenly shared between atoms
• Molecular polarity


  Whether the whole molecule is polar or nonpolar
• Molecules to categorise


  • PCl3
  • BH3
  • CH3Cl
  • SO2
  • SO3
  • H2O
  • NH3
  • HCN
  • CH4
  • CH2O
  • O3
  • CH3CH3
  • OF2
  • N2
• VSEPR model


  1. Rationalizes molecular geometries based on repulsions between electron domains

  2. Bonding pairs and nonbonding pairs of electrons create electron domains

  3. Electron domains orient to minimize electrostatic repulsions

  4. Arrangement of electron domains is the molecular geometry

  5. Arrangement of atoms is the molecular shape
• Electronegativity


  Relative ability of an element to attract electrons in a chemical bond<|>Increases across a period<|>Decreases down a group