GEN CHEM

• Covalent molecular compounds


  Compounds formed by the sharing of electrons between atoms
• Nonmetallic elements have a covalent molecular structure
• Properties of covalent molecular compounds


  • Usually have low melting point and boiling points

  Are nonconductors of electricity
• Ionic compounds


  Compounds formed by the electrostatic attraction between oppositely charged ions (cations and anions)
• Properties of ionic compounds


  • Have very high melting and boiling points

  Are nonconductors of electricity in solid state but conductors in molten/liquid state and aqueous solution

  Consist of anions and cations arranged in a well-ordered array
• Octet rule


  Most representative/main group elements form ions by gaining, sharing or losing electrons to reach the stable valence electron configuration of the noble gases
• Lewis structure


  Indicates the number of valence electrons in an atom represented by dots scattered on four sides of the atomic symbol
• Formation of a covalent bond

  Sharing of electrons between atoms

  One pair of shared electrons forms one covalent bond
• Bonding pair


  Pair of shared electrons in a covalent bond
• Lone pair


  Pair of electrons that do not participate in bond formation
• Bond length decreases as the number of shared electron pairs increases
• Molecular formula


  Chemical formula for covalent compounds, referring to separate, discrete molecules
• Writing and naming formula of molecular compounds

  Begin with the nonmetal closest to the lower left corner of the periodic table (except hydrogen)

  List other nonmetal symbols with numerical subscripts if more than one

  Use common names for some simple covalent compounds

  For binary covalent compounds, name the first element and add "-ide" to the second element name, using numerical prefixes
• Phosphorus trichloride


  Written as PCl3
• For some simple covalent compounds, we use common names rather than systematic names
• Examples of common names for simple covalent compounds

  • H2O: water
  • NH3: ammonia
  • CH4: methane
• Naming binary (two-element) covalent compounds


  • The first element in the formula is simply listed using the name of the element
  • The second element is named by taking the stem of the element name and adding the suffix -ide
  • A system of numerical prefixes is used to specify the number of atoms in a molecule
  • If the second element is oxygen, the trailing vowel is usually omitted from the end of a polysyllabic prefix but not a monosyllabic one
• Covalent bond formation


  Involves only the electrons in an atom's outermost energy levels, or the valence electrons
• Electron-dot-notation


  An electron-configuration notation in which only the valence electrons of an atom of a particular element are shown, indicated by dots placed around the element's symbol
• Lewis structures


  Formulas in which atomic symbols represent nuclei and inner-shell electrons, dot-pairs or dashes between two atomic symbols represent electron pairs in covalent bonds and dots adjacent to only one atomic symbol represent the unshared electrons
• Steps in Writing the Lewis Dot Structure

  1. Determine the total number of valence electrons in the atoms to be combined

  2. Arrange the atoms to form a skeleton structure for the formula, with the least-electronegative atom as the central atom

  3. Put the rest of the electrons in pairs around the atoms bonded to the central atom until each atom, except hydrogen, has four pairs around it to satisfy the octet rule

  4. Move the electrons to form double or triple bonds in order that all atoms have four pairs, except for hydrogen which should have only one
• 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
• Polarity of bonds


  • H-F bond is more polar than H-Cl bond
• When the electronegativity difference between two atoms is very large, an ionic bond is formed instead of a covalent bond
• Classification of bonds based on electronegativity difference


  • Ionic: >2.0
  • Polar Covalent: 0.5 - 1.9
  • Nonpolar Covalent: <0.4