16 - Valence Bond Theory SCH4U – Structure and Properties of Matter

Date: ______

Valence Bond Theory

  • Developed by chemists Walter Heitler and Fritz London in 1927
  • Proposed that a ______forms when ______from two atoms ______
  • The two overlapping orbitals form one “______” which is a volume of space between two atomic nuclei in which there is a high probability of finding the now ______

  • Consider the electron orbital notation for Fluorine
  • There is one empty space in the p-orbitals
  • If two fluorine combine, they can share the single electrons

  • Let’s look at water (H2O)

  • A good bonding theory will explain all of the properties of the molecule’s structure including: ______, bond energy, ______and ______
  • Problem: for most molecules, the Valence Bond theory does not match ______observations
  • Consider Water
  • The bonds in H2O would involve the oxygen “_____” orbitals overlapping with the hydrogen “_____” orbital
  • As a result, an expected bond angle of ______
  • Experimental observations show that the bond angle in water is ______

  • Consider methane (CH4)
  • The carbon atom would be using “_____” and “_____” orbitals to form bonds
  • The energy of “_____” orbital electrons is higher than the energy of “____” orbital electrons, this should be reflected in the bond energies
  • In the real molecule all bond energies are ______
  • Also, this combination of orbitals can’t explain the tetrahedral shape of this molecule with bond angles of ______

Orbital Hybridization Theory

  • Linus Pauling proposed:
  • when bonding, the “______” atomic orbitals of the central atom are replaced with new “______”
  • “s orbital” and “p orbitals” are reconfigured into “______hybrid orbitals”
  • hybridizing electrons from one “____” orbital and three “____” orbitals results in four identical hybrid orbitals called “______hybrid orbitals” that are arranged in a tetrahedral pattern around the nucleus
  • hybrid orbitals are ______in energy to the original “pure” orbitals

Sigma Bonds (σ)

  • A sigma bond is formed when two orbitals overlap in a direct “______” fashion
  • The “______” formed by this type of overlap lies directly between and in the same plane as the two atomic ______
  • This very effective overlap results in a very ______

Pi Bonds (π)

  • A pi bond is formed by the less effective “______” overlap of two “____” orbitals
  • This results in the bonding orbital forming above and below the plane of the ______
  • This less effective orbital overlap results in the pi bond being ______than the sigma bond

Page 4 of 4