valence bond theory

Examples of valence bond theory in the following topics:

• Bonding in Coordination Compounds: Valence Bond Theory

  • Valence bond theory is used to explain covalent bond formation in many molecules.
  • Valence bond theory is a synthesis of early understandings of how chemical bonds form.
  • Finally, Linus Pauling integrated Lewis' proposal and the Heitler-London theory to give rise to two additional key concepts in valence bond theory: resonance and orbital hybridization.
  • It is in this aspect of valence bond theory that we see the concept of resonance.
  • Calculate the theoretical hybridization of a metal in a coordination complex based on valence bond theory

• Explanation of Valence Bond Theory

  • Valence bond theory states that overlap between two atomic orbitals forms a covalent bond between two atoms.
  • In chemistry, valence bond (VB) theory is one of two basic theories—along with molecular orbital (MO) theory—that use quantum mechanics to explain chemical bonding.
  • According to VB theory, a covalent bond forms from the physical overlap of half-filled valence orbitals in two atoms.
  • This theory is used to explain the covalent bond formation in many molecules.
  • To complete their valence shells, they bond and share one electron with each other.

• Bond Order

  • Bond order is the number of chemical bonds between a pair of atoms.
  • Bond order indicates the stability of a bond.
  • In molecular orbital theory, bond order is also defined as the difference, divided by two, between the number of bonding and antibonding electrons; this often, but not always, yields the same result.
  • Bond order is also an index of bond strength, and it is used extensively in valence bond theory.
  • A bond order of zero is obtained by placing the available electrons in the bonding and antibonding levels, indicating that dihelium does not exist according to valence bond and bond order theory.

• Hybridization in Molecules Containing Double and Triple Bonds

  • sp2, sp hybridizations, and pi-bonding can be used to describe the chemical bonding in molecules with double and triple bonds.
  • Hybridized orbitals are very useful in explaining of the shape of molecular orbitals for molecules, and are an integral part of valence bond theory.
  • Ethene (C2H4) has a double bond between the carbons.
  • The chemical bonding in acetylene (ethyne) (C2H2) consists of sp-sp overlap between the two carbon atoms forming a sigma bond, as well as two additional pi bonds formed by p-p overlap.
  • In ethene, carbon sp2 hybridizes, because one π (pi) bond is required for the double bond between the carbons, and only three σ bonds form per carbon atom.

• Physical Properties of Covalent Molecules

  • The Lewis bonding theory can explain many properties of compounds.
  • Lewis bonding theory states that these atoms will share their valence electrons, effectively allowing each atom to create its own octet.
  • Lewis theory also accounts for bond length; the stronger the bond and the more electrons shared, the shorter the bond length is.
  • According to the theory, triple bonds are stronger than double bonds, and double bonds are stronger than single bonds.
  • However, the theory implies that the bond strength of double bonds is twice that of single bonds, which is not true.

• The Shape of Molecules

  • Bonding configurations are readily predicted by valence-shell electron-pair repulsion theory, commonly referred to as VSEPR in most introductory chemistry texts.
  • In the three examples shown above, the central atom (carbon) does not have any non-bonding valence electrons; consequently the configuration may be estimated from the number of bonding partners alone.
  • In each case there are four regions of electron density associated with the valence shell so that a tetrahedral bond angle is expected.
  • The compound boron trifluoride, BF3, does not have non-bonding valence electrons and the configuration of its atoms is trigonal.
  • Nice treatments of VSEPR theory have been provided by Oxford and Purdue.

• Table of Geometries

  • The VSPER theory detremines molecular geometries (linear, trigonal, trigonal bipyramidal, tetrahedral, and octahedral).
  • The valence shell electron pair repulsion (VSEPR) model focuses on the bonding and nonbonding electron pairs present in the outermost (valence) shell of an atom that connects with two or more other atoms.
  • The orbitals containing the various bonding and non-bonding pairs in the valence shell will extend out from the central atom in directions that minimize their mutual repulsions.
  • A table of geometries using the VSEPR theory can facilitate drawing and understanding molecules.
  • The bonded angles in the table are ideal angles from the simple VSEPR theory; the actual angle for the example given is in the following column.

• Atomic and Molecular Orbitals

  • A more detailed model of covalent bonding requires a consideration of valence shell atomic orbitals.
  • The valence shell electron configuration of carbon is 2s2, 2px1, 2py1 & 2pz0.
  • In this case, the valence shell would have six electrons- two shy of an octet.
  • In order to explain this covalent bonding, Linus Pauling proposed an orbital hybridization model in which all the valence shell electrons of carbon are reorganized.
  • Thus, the four covalent bonds of methane consist of shared electron pairs with four hydrogen atoms in a tetrahedral configuration, as predicted by VSEPR theory.

• Covalent Bonds

  • A fluorine atom has seven valence electrons.
  • Carbon will then have five valence electrons (its four and the one its sharing with fluorine).
  • Covalent bonds are a class of chemical bonds where valence electrons are shared between two atoms, typically two nonmetals.
  • A fluorine atom has seven valence electrons.
  • Carbon will then have five valence electrons (its four and the one its sharing with fluorine).

• Applying the VSEPR Model

  • The valence shell electron pair repulsion (VSEPR) model predicts the shape of individual molecules based on the extent of electron-pair electrostatic repulsion.
  • The number of atoms bonded to a central atom combined with the number of pairs of its nonbonding valence electrons is called its steric number.
  • This makes no difference to VSEPR theory; the central carbon atom is still joined to two other atoms, and the electron clouds that connect the two oxygen atoms are 180° apart.
  • The bonding geometry will not be tetrahedral when the valence shell of the central atom contains nonbonding electrons.
  • Lots and lots of practice problems for VSEPR theory.