Molecular Orbital Theory Valence electrons are delocalized Electrons

Molecular Orbital Theory • Valence electrons are delocalized. • Electrons occupy molecular orbitals that spread throughout the molecule. Valentim Nunes, Engineering Unit of IPT February, 2018

Experience shows that O 2 is paramagnetic O O All the electrons are paired! O 2 should be diamagnetic! Molecular orbitals theory — chemical bonds results from interactions between atomic orbitals to form molecular orbitals.

The Paramagnetism of O 2

• Bonding and antibonding sigma molecular orbitals forms from 1 s adjacent orbitals.

Levels of energy of the bonding and antibonding molecular orbitals of hydrogen (H 2). The bonding molecular orbital has lower energy and grater stability than the correspondent atomic orbitals. The antibonding molecular orbital has higher energy and lower stability than the atomic orbitals.

1. Number of MO = number of atomic orbitals utilized. 2. Electrons are distributed accordingly with the Aufbau principle.

Does the Dihelium molecule, He 2, exists?

Sigma bond from p orbitals

Π bonds from p orbitals Sideways overlap of atomic 2 p orbitals that lie in the same direction in space give π bonding and antibonding MOs.

General Diagram of energy levels of molecular orbitals for homo nuclear diatomic molecules of the second period elements : Li 2, Be 2, B 2, C 2 e N 2.

Configuration of the Molecular orbitals (OM) • • • The number of molecular orbitals (OM) formed is always equal to the number of atomic orbitals. The more stable a bonding OM, the less stable is the correspondent anti-bonding OM. The filling of the OM is by order of energies (Aufbau principle). Each OM can be occupied by the maximum of 2 electrons (with different spin – Pauli Principle). The Hund Rule is applied when we have OM of the same energy. The number of electrons in the OM its equal to the sum of all the electrons in the atoms involved in the chemical bond.

Bond order = 1 2 ( N. º electrons in bonding OM - ) N. º electrons in antibonding OM

Delocalized molecular orbitals are not confined in the space of two atoms but can spread over several atoms.

Electronic density above and below the plan in the molecule of benzene.

Bonding in the carbonate ion

Carbon