GEOMETRY OF MOLECULES NIKAM N D DEPARTMENT OF

GEOMETRY OF MOLECULES NIKAM N. D. DEPARTMENT OF CHEMISTRY

Rules for Predicting Molecular Geometry 1. Sketch the Lewis structure of the molecule or ion 2. Count the electron pairs and arrange them in the way that minimizes electron-pair repulsion. 3. Determine the position of the atoms from the way the electron pairs are shared. 4. Determine the name of the molecular structure from the position of the atoms. 5. Double or triple bonds are counted as one bonding pair when predicting geometry.

Note: The same rules apply for molecules that contain more than one central atom

The Dipole A dipole arises when two electrical charges of equal magnitude but opposite sign are separated by distance.

The dipole moment (m) m= Qr where Q is the magnitude of the charges and r is the distance

For a polyatomic molecule we treat the dipoles as 3 D vectors The sum of these vectors will give us the dipole for the molecule

Overlap of Orbitals

The degree of overlap is determined by the system’s potential energy equilibrium bond distance The point at which the potential energy is a minimum is called the equilibrium bond distance

Formation of sp hybrid orbitals The combination 2 s of an s orbital and a p orbital produces 2 new orbitals called sp orbitals. These new orbitals are called hybrid orbitals The process is called hybridization What this means is that both the s and one p orbital are involved in bonding to the connecting atoms

Formation of sp 2 hybrid orbitals

Formation of sp 3 hybrid orbitals

Hybrid orbitals can be used to explain bonding and molecular geometry

Multiple Bonds Everything we have talked about so far has only dealt with what we call sigma bonds Sigma bond (s) A bond where the line of electron density is concentrated symmetrically along the line connecting the two atoms.

Pi bond (p) A bond where the overlapping regions exist above and below the internuclear axis (with a nodal plane along the internuclear axis).

Example: H 2 C=CH 2

Example: H 2 C=CH 2

Example: HC CH

Delocalized p bonds When a molecule has two or more resonance structures, the pi electrons can be delocalized over all the atoms that have pi bond overlap.

Example: C 6 H 6 benzene Benzene is an excellent example. For benzene the p orbitals all overlap leading to a very delocalized electron system In general delocalized p bonding is present in all molecules where we can draw resonance structures with the multiple bonds located in different places.

Moleculuar Orbital (MO) Theory ANTBONDING These two new orbitals have different energies. BONDING The one that is lower in energy is called the bonding orbital, The one higher in energy is called an antibonding orbital.

Energy level diagrams / molecular orbital diagrams

MO Theory for 2 nd row diatomic molecules Molecular Orbitals (MO’s) from Atomic Orbitals (AO’s) 1. # of Molecular Orbitals = # of Atomic Orbitals 2. The number of electrons occupying the Molecular orbitals is equal to the sum of the valence electrons on the constituent atoms. 3. When filling MO’s the Pauli Exclusion Principle Applies (2 electrons per Molecular Orbital) 4. For degenerate MO’s, Hund's rule applies. 5. AO’s of similar energy combine more readily than ones of different energy 6. The more overlap between AOs the lower the energy of the bonding orbital they create and the higher the energy of the antibonding orbital.

Example: Li 2

MOs from 2 p atomic orbitals s p 1) 1 sigma bond through overlap of orbitals along the internuclear axis. 2) 2 pi bonds through overlap of orbitals above and below (or to the sides) of the internuclear axis.

Interactions between the 2 s and 2 p orbitals The s 2 s and s 2 p molecular orbitals interact with each other so as to lower the energy of the s 2 s MO and raise the energy of the s 2 p MO.

For B 2, C 2, and N 2 the interaction is so strong that the s 2 p is pushed higher in energy than p 2 p orbitals

Paramagnetism of O 2