Gen chem 23

• Covalent bonding


  Atoms joined by sharing electrons
• Nonmetallic elements


  • Have a covalent molecular structure

  The combination of two nonmetals produces a compound with a covalent molecular structure
• Properties of covalent molecular compounds


  • Usually have low melting point and boiling points

  Are nonconductors of electricity
• Ionic compounds


  Formed by the electrostatic attraction that holds together the oppositely charged ions, the cations and anions
• Properties of ionic compounds


  • Have very high melting and boiling points

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

  Consist of anions and cations arranged in a well-ordered array in the solid state

  The ionic bond formed is very strong
• Octet rule


  Most representative or main group elements form ions by either gaining, sharing or losing electrons, in such a manner as 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
• Covalent bond


  Formed by the sharing of electrons between atoms
• Covalent molecular compounds


  • Can have single, double or triple bonds

  Bond length decreases as the number of shared electron pairs increases
• Molecular formula


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


  Typically begins with the nonmetal closest to the lower left corner of the periodic table, except hydrogen

  Numerical subscripts are used if there is more than one of a particular atom

  For simple covalent compounds, common names are used rather than systematic names

  Naming binary covalent compounds is similar to naming simple ionic compounds, using the stem of the element name and adding the suffix -ide, with numerical prefixes to specify the number of atoms
• 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.
• Normally, no prefix is added to the first element's name if there is only one atom of the first element 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


  Usually 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

  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