Antiorbitals &Sigma &Pi 11/chemistry/4-chemical-bonding-and- molecular-structurehttp://textbook.s-anand.net/ncert/class- 11/chemistry/4-chemical-bonding-and- molecular-structure

Orbitals share a region of space, they overlap The overlap of orbitals allows two electrons of opposite spin to share the common space between the nuclei, forming a covalent bond.

Sigma bonds (symbol: σ) A sigma bond is formed when two atomic orbitals on different atoms overlap along a line drawn through the two nuclei. ( head-on) The first covalent bond formed between two atoms is always a sigma bond.

Pi bonds This bond results from the sideways overlap of parallel p orbitals.

CH 4 and C 2 H 2

S.G page 27 # 15 HL Workbook # 14

Combination of Atomic Orbitals

Molecular Orbital Theory When two atomic orbitals from different atoms combine constructively, the e density increases resulting in a molecular orbital with less energy than the two orbitals: bonding molecular orbital(electrons will tend to fill lower energy levels) If the two orbitals combine destructively, the electron density decreases and the molecular orbital will have higher energy: anti-bonding orbital*(electrons will stay at their orbitals)

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

Energy level diagrams / molecular orbital diagrams This model explains why hydrogen forms a diatomic molecule when the 1s orbitals combine whereas helium is monoatomic.

Hybridization Hybridization: a model that describes the changes in the atomic orbitals of an atom when it forms a covalent compoundatom

HYBRIDISATION OF ORBITALS The electronic configuration of a carbon atom is 1s 2 2s 2 2p 2 1 1s 2 2s 2p

HYBRIDISATION OF ORBITALS The electronic configuration of a carbon atom is 1s 2 2s 2 2p 2 1 1s 2 2s 2p If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s 2 2s 1 2p 3 1 1s 2 2s 2p The process is favourable because of the arrangement of electrons; four unpaired and with less repulsion is more stable

MOs from 2p atomic orbitals 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.  

Hybrid orbitals can be used to explain bonding and molecular geometry

Formation of sp 2 hybrid orbitals

Formation of sp 3 hybrid orbitals

2s 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 hybrid orbitals The combination of an s orbital and a p orbital produces 2 new orbitals called sp orbitals.

Example: H 2 C=CH 2

Example: HC  CH