Covalent Bonding

• Covalent bonding


  Chemical bonding resulting from two nuclei attracting the same shared electrons
• Covalent compounds


  Compounds that contain only covalent bonds
• Hydrogen molecule (H2)


  • Simplest of all molecules
  • Orbitals that contain the valence electrons overlap to create an orbital common to both atoms
  • Most favorable location for the shared electrons is the area directly between the two nuclei
• Octet rule


  Atoms in a covalent compound would attain eight electrons (octet) in its valence shell
• Bonding electrons


  • Pairs of valence electrons that are shared between atoms in a covalent bond
• Nonbonding electrons


  • Pairs of valence electrons on an atom that are not involved in electron sharing
• Single covalent bond


  Covalent bond in which two atoms share one pair of electrons
• Double covalent bond


  Covalent bond in which two atoms share two pairs of electrons
• Triple covalent bond


  Covalent bond in which two atoms share three pairs of electrons
• Predicting formula of simple covalent compounds

  1. Write the Lewis symbol of each atom

  2. Determine how many atoms of each element are needed to complete the octet of the atoms involved
• Numerical prefixes in names of binary molecular compounds

  Give the number of each type of atom present
• Lewis structure of HCN


  • Hydrogen has 1 valence electron, carbon has 4 valence electrons, nitrogen has 5 valence electrons
  • Draw the molecular skeleton with single covalent bonds
  • Add nonbonding electron pairs to the structure such that the atoms bonded to the central atom have octets
• Drawing Lewis structure

  1. Step 1: Calculate total number of valence electrons

  2. Step 2: Draw molecular skeleton with single covalent bonds

  3. Step 3: Add nonbonding electron pairs to give each atom an octet

  4. Step 4: Check if central atom has octet, if not proceed to Step 5

  5. Step 5: Adjust bonding to give central atom an octet

  6. Step 6: Verify total number of electrons matches initial calculation
• Ionic compounds with polyatomic ions

  Covalent bonding within polyatomic ion, ionic bonding between ion and opposite charge ions<|>Lewis structure drawn separately for positive and negative ions
• Drawing Lewis structure of polyatomic ion

  1. Step 1: Calculate total valence electrons including charge

  2. Step 2: Draw molecular skeleton

  3. Step 3: Add nonbonding pairs to give atoms octets

  4. Step 4: No more electrons needed if central atom has octet
• Formal charge


  Charge an atom would have if all atoms had same electronegativity<|>Formal charge = [valence e-] - [(lone pair e-) + (1/2 bonding e-)]<|>Choose structure with formal charges closest to zero<|>Negative charge on more electronegative atom
• Exceptions to octet rule

  • Odd number of electrons
  • Less than octet of valence electrons
  • More than octet of valence electrons
• Covalent compounds


  • Low melting and boiling points
  • Non-conductors of electricity
  • Can be solids, liquids or gases at room temperature
• Formal charges do not represent real charges on atoms, just a bookkeeping convention
• Molecular geometry


  The three-dimensional arrangement of atoms within a molecule
• Molecular geometry


  • It is an important factor in determining the physical and chemical properties of a substance
• Using VSEPR theory to predict molecular geometry

  1. Draw the Lewis structure

  2. Determine the number of VSEPR electron groups around the central atom

  3. Identify each as a bonding electron group or a nonbonding electron group

  4. Use the table to determine the molecular geometry
• VSEPR electron group (or electron domain)


  A collection of valence electrons present in a localized region about the central atom in a molecule
• Electron-group geometry


  The three-dimensional arrangement of the electron groups in space
• Electronegativity


  A measure of the relative attraction that an atom has for the shared electrons in a bond
• From left to right across periods, electronegativity values increase
• From top to bottom within a group, electronegativity values decrease
• Bond polarity


  A measure of the degree of inequality in the sharing of electrons between two atoms in a chemical bond
• Nonpolar covalent bond


  A covalent bond in which there is equal sharing of electrons between two atoms
• Polar covalent bond


  A covalent bond in which there is unequal sharing of electrons between two atoms
• Molecular polarity


  A measure of the degree of inequality in the attraction of bonding electrons to various locations within a molecule
• Polar molecule


  A molecule in which there is an unsymmetrical distribution of electronic charge
• Nonpolar molecule


  A molecule in which there is a symmetrical distribution of electron charge
• Examples of nonpolar molecules


  • CO2
  • BF3
  • CCl4
  • PF5
  • SF6
  • XeF4
• Molecular polarity depends on two factors: molecular geometry and bond polarities
• The presence of polar covalent bonds in a molecule is not an indication that the molecule is polar as a whole
• For a linear molecule with polar bonds


  The polarity effects cancel each other
• For a bent (angular) molecule with polar bonds


  The bond polarities do not cancel one another
• For a linear molecule with polar bonds


  Both bond polarities contribute to the molecule being polar
• Molecules with four or five atoms commonly have trigonal planar and tetrahedral geometries