14.2.1.Antiorbitals &Sigma &Pi 11/chemistry/4-chemical-bonding-and- molecular-structure

14.2.1.Antiorbitals &Sigma &Pi

• http://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.

CH4 and C2H2

• 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) TheoryMolecular Orbital (MO) Theory

Energy level diagrams / molecular Energy level diagrams / molecular orbital diagramsorbital diagrams

• This model explains why hydrogen forms a diatomic molecule when the 1s orbitals combine

whereas helium is monoatomic.

HybridizationHybridization: a model that describes the

changes in the atomic orbitals of an atom when it forms a covalent compound

• http://ibchem.com/IB/ibnotes/full/bon_htm/14.2.htm

HYBRIDISATION OF ORBITALSHYBRIDISATION OF ORBITALS

The electronic configuration of a carbon atom is 1s22s22p2

1 1s

22s

2p

HYBRIDISATION OF ORBITALSHYBRIDISATION OF ORBITALS

The electronic configuration of a carbon atom is 1s22s22p2

1 1s

22s

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 1s22s12p3

1 1s

22s

2p

The process is favourable because of the arrangement of electrons; four unpaired and with less repulsion is more stable

MOs from 2p atomic orbitalsMOs 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 Hybrid orbitals can be used to explain bonding and molecular geometryand molecular geometry

Formation of spFormation of sp22 hybrid orbitalshybrid orbitals

Formation of spFormation of sp33 hybrid orbitalshybrid 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 orbitalsFormation of sp hybrid orbitals

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

Example: HExample: H22C=CHC=CH22

Example: HExample: H22C=CHC=CH22

Example: Example: HCHCCHCH