week 4

• Molecular geometry


  The three-dimensional arrangement of a molecule's atoms in space
• Polarity


  The uneven distribution of molecular charge
• VSEPR Theory


  • States that the best arrangement of a given number of shared and unshared electrons is the one that minimizes the repulsion among them, resulting in a particular geometric shape of a molecule
• Basic Electron-Group Geometries


  • Linear

  Trigonal planar

  Tetrahedral

  Trigonal bipyramidal

  Octahedral
• Predicting Molecular Geometry


  Draw the appropriate Lewis structure

  2. Determine the number of electron groups around the central atom and identify each as a bonding pair or lone pair

  3. Determine the molecular geometry from the table
• Molecular Geometry and Bond Angle of Compounds


  • Linear (0 lone pairs, 2 bonding pairs, 180° bond angle)

  Trigonal planar (0 lone pairs, 3 bonding pairs, 120° bond angle)

  Angular (1 lone pair, 2 bonding pairs, 120° bond angle)

  Tetrahedral (0 lone pairs, 4 bonding pairs, 109.5° bond angle)

  Trigonal pyramidal (1 lone pair, 3 bonding pairs, 107° bond angle)

  Angular (2 lone pairs, 2 bonding pairs, 104.5° bond angle)

  Trigonal bipyramidal (0 lone pairs, 5 bonding pairs, 90°, 120°, 180° bond angles)

  Seesaw (1 lone pair, 4 bonding pairs, 90°, 120°, 180° bond angles)

  1. shaped (2 lone pairs, 3 bonding pairs, 90° bond angle)

  Linear (3 lone pairs, 2 bonding pairs, 180° bond angle)

  Octahedral (0 lone pairs, 6 bonding pairs, 90°, 180° bond angles)

  Square pyramidal (1 lone pair, 5 bonding pairs, 90° bond angle)

  Square planar (2 lone pairs, 4 bonding pairs, 90° bond angle)
• Valence Bond Theory


  Explains how covalent bonds are formed through the overlap of atomic orbitals
• A polar bond has a negative end and a positive end
• Electronegativity


  The ability of an atom in a molecule to attract shared electrons
• The greater the difference in electronegativity between two atoms, the more polar the bond
• If the difference in electronegativity is greater than or equal to 2.0, there would be a transfer of electrons rather than a sharing of electrons and an ionic bond is formed
• Electronegativity (EN)


  A property that indicates an atom's ability to attract shared electrons in a molecule
• The greater the electronegativity of an atom, the stronger is its ability to attract shared electrons
• Linus Pauling developed the first and most widely used electronegativity scale
• Polarity of a bond


  Indicated by the difference in electronegativity between the atoms forming the bond
• The greater the difference in electronegativity


  The more polar the bond
• 1. F bond vs H-Cl bond


  • H-F bond (ENF = 4.0, ENH = 2.1) is more polar than H-Cl bond (ENCl = 3.2)
• When the electronegativity difference between two atoms is very large, electron transfer occurs and an ionic bond is formed instead of a covalent bond
• Classifying bonds based on electronegativity difference

  • Ionic: >2.0
  • Polar Covalent: 0.5 - 1.9
  • Nonpolar Covalent: <0.4
• The presence of polar covalent bonds in a molecule does not necessarily mean the molecule as a whole is polar
• If the geometry of a molecule is such that the polarity of the bonds cancels each other


  The molecule becomes nonpolar
• CO2 vs CHCl3

  • CO2 is nonpolar, CHCl3 is polar
• Molecules of high symmetry like BF3, CH4, PF5, SF6 are nonpolar even though the bonds are polar
• Molecules of less geometric symmetry like H2S and NH3 can be polar even when all bond moments are identical