Part 1: Molecular Orbitals Bonding Theories Part 1: Molecular Orbitals

Objectives Describe the relationship between atomic and molecular orbitals

Important Vocabulary Molecular orbitals Bonding orbital Sigma bond Pi bond

Molecular Orbitals The model for covalent bonding you have been using assumes that the orbitals are those of the individual atoms There is a quantum mechanical model of bonding that describes the electrons in molecules using orbitals that exist only for groupings of atoms Molecular orbitals form when two atoms combine and their atomic orbitals overlap

Forming Molecular Orbitals Just as an atomic orbital belongs to a particular atom, a molecular orbital belongs to a molecule as a whole 2 electrons are required to fill a molecular orbital just like an atomic orbital A molecular orbital that can be occupied by 2 electrons of a covalent bond is called a bonding orbital

Formation of Molecular Orbitals

Sigma Bonds When two atomic orbitals combine to form a molecular orbital that is symmetrical around the axis connecting two atomic nuclei, a sigma bond is formed The symbol for this bond is the Greek letter sigma (σ)

Repulsion Forces In general, covalent bonding results from an imbalance between the attractions and repulsions of the nuclei and electrons involved Because their charges have the same sign, nuclei repel other nuclei and electrons repel other electrons

Sigma Bonds with p orbitals Atomic p orbitals can also overlap to form molecular orbitals They overlap end-to-end

Pi Bonds In some molecules, orbitals can overlap side-by-side. This side-by-side overlap of atomic p orbitals produces a pi molecular orbital When a pi molecular orbital is filled with 2 electrons, a pi bond results In a pi bond, the bonding electrons are most likely to be found in sausage-shaped regions above and below the bond axis of the bonded atoms It is represented by the Greek letter, π It is not symmetrical around the bond axis , the pi orbitals bonding overlaps less than sigma bond Thus, pi bonds are weaker than sigma bonds

Pi Bonds

Why are pi bonds weaker than sigma bonds? The greater the extent of overlapping, the higher the probability of finding the valence electrons in between the nuclei The overlap in sigma bonds is greater than that in pi bonds and hence the bond will be stronger & shorter. Because of this difference in strength, a double bond is not twice as strong as a single bond (which is sigma) because one of them will be a pi bond.

Determining Molecular Shapes Three-dimensional shape is important in determining the molecule’s physical and chemical properties For example:

Lewis Structures & Molecular Shape A molecule made of only two atoms has a linear shape only Molecules with more than two molecules are not as easy to determine Chemists must use a model to predict the shape of these molecules This model is based on valence shell electron pair repulsion theory (VSEPR)