VSEPR

• Molecular geometry describes the shape formed by just the atoms in the molecule
• VSEPR Theory

  Analyze a structure's electron domains to predict the shape around the central atom
• Chemical behavior of molecules

  Determined by their shape and polarity
• Electrons in bonds or lone pairs are grouped into electron domains
• Atoms with all bonded electron domains have the same electron and molecular geometry
• Each additional lone pair of electrons occupies more space than bonding electrons, causing increased repulsion and decreased bond angles
• Lewis structures

  Show how valence electrons are organized around the atoms within a molecule, around the central atom
• Covalent bonds


  Form when non-metals share electrons to achieve a stable electron configuration, compounds formed are called molecules
• Electron domains arrangement

  Constantly repelling each other, pushing to arrange themselves in three-dimensional space to minimize repulsion and maximize bond angles
• Impact of lone pairs on bond angles
• Molecules with four electron domains

  • Methane, ammonia, water
• Electron geometry describes the shape of all electron domains in three-dimensional space
• Molecular geometries with three electron domains

  Methanal and Ozone
• Ozone


  • Three electron domains, resulting in a bent shape with a bond angle of about 117° due to increased repulsion of lone pair of electrons
• Each additional lone pair of electrons occupies more space than electrons stored in bonds

  Increased repulsion with bonding electrons pushes bonds inward, decreasing bond angles
• Molecular geometry trees

  • For three and four electron domains, and two electron domains
• Water has the smallest bond angle


  Due to the most non-bonded pairs of electrons
• Individual polar bonds don't always cause an entire molecule to have a dipole moment
• Bonds in a covalent compound are classified as polar or non-polar based on their differences in electronegativity
• Carbon dioxide


  • Linear molecular geometry with a bond angle of 180° to achieve maximum separation between the two electron domains
• Non-polar molecules

  • Carbon tetrachloride, Carbon dioxide
• Carbon tetrachloride and carbon dioxide are considered non-polar despite having polar bonds
• Molecular geometries

  • Trigonal planar, Bent
• Molecular geometry plays a large role in a molecule's overall polarity


  And how it interacts with other compounds
• Polar bonds create partial positive and negative charges called Bond dipoles
• Structures with different bond angles

  • Methane, Ammonia, Water
• Methane has the largest bond angles


  Each of its domains are bonded to
• Methanal


  • Central carbon with three electron domains, creating bond angles of 120° and a flat triangle shape
• Carbon dioxide


  • CO2
• The presence and orientation of polar bonds determined by the molecule's geometry will cause bond dipoles to either cancel out or add together
• Polar molecules

  • Trichloromethane (Chloroform)
• Chloroform is a polar molecule due to the replacement of one chlorine atom in carbon tetrachloride with a hydrogen atom
• Dipole moment in methanol


  Overall dipole moment pointing towards oxygen
• Molecular dipole in trichloromethane


  Overall dipole moment in red
• Diamond


  • Each carbon atom bonded to four others in a tetrahedral geometry, hardest substance known, high melting point
• Lone pair of electrons in ammonia

  Exaggerated movement away from bonding electrons due to stronger pull of nitrogen relative to hydrogen
• Repulsion between bonded and non-bonded valence electrons in carbon structures


  Causes molecules to take shapes known as their electron and molecular geometry
• Allotropes of carbon

  • Graphite
  • Diamond
  • Fullerene
• Carbon-hydrogen bond


  Generally considered non-polar due to small difference in electronegativity between carbon and hydrogen
• Learning about the geometry and polarity of covalent compounds is vital to understanding the chemical behavior of covalent molecules and the intermolecular forces that attract them to each other