Bonding

Lecture 2

Valence bond theory

A covalent bond forms from the overlap of two atomic orbitals, pairing electrons attraccted to both nuclei

What type of bond forms from head-on overlap of orbitals?

A sigma (σ) bond

Example of a simple σ bond

H-H bond from overlap of two H 1s orbitals

What happens to electrons in overlapping atomic orbitals?

The electrons pair up and are attracted to the nuclei of both atoms, bonding the two atoms together

What are molecular orbitals?

Orbitals occupied by electrons in a molecule

How are molecular orbitals formed?

By the linear combination of atomic orbitals

What results from the linear combination of two atomic orbitals?

One bonding molecular orbital and one antibonding molecular orbital

In H₂ where do the electrons reside?

In the lowest energy molecular orbital, which is the σ-bonding orbital

What is the σ* orbital?

An antibonding molecular orbital that remains empty in stable molecules like H₂

s-orbitals combine to give which molecular orbital?

σ-bonding orbital and σ*-anti bonding orbital

p-orbitals combine to form which molecular orbital?

π-bonding orbital and π*-anti bonding orbital

How many valence electrons does carbon have?

Four (2s²2p²)

Why does carbon hybridise its orbitals?

To form equivalent bonds with specific geometries that are better oriented for bonding

Who introduced the concept of hybridisation?

Linus Pauling

How to determine hybridisation from bonding

Count electron regions around the atom

• A single bond

• A double bond (counts as one region)

• A triple bond (counts as one region)

• a lone pair

If there are 4 electron regions, which hybridisation is involved and what is the shape of the molecule?

sp³ and tetrahedral

If there are 3 electron regions, which hybridisation is involved and what is the shape of the molecule?

sp² and trigonal planae

If there are 2 electron regions, which hybridisation is involved and what is the shape of the molecule?

sp and linear

Explain the concept of sp³ hybridisation

• Carbon has one 2s orbital and three 2p orbitals

• The atom mixes its s and p orbitals creating new hybrid orbitals which are equal in energy, point in specific directions and overlap better with other atoms, thus a stronger bond

• Four sp³ orbitals are formed

• For each C-H bond, one sp³ orbital from carbon overlaps with one 1s bond from hydrogen, forming a σ bond

• The four sp³ orbitals repel each other and spread out as far as possible in 3D space, thus resulting in a tetrahedral shape with bind angle, 109.5°

What orbitals combine to form sp³ hybrid orbitals in methane and ethane?

One s orbital and three p orbitals

How many sp³ hybrid orbitals are formed in methane and ethane?

Four

Why are all the C-H bond in methane identical?

Because carbon uses four equivalent sp³ hybrid orbitals

Why are sp³ orbitals stronger for pure bonding than pure s or p orbitals?

Their larger lobe overlaps more effectively with other orbitals

What is the geometry of ethane?

Tetrahedral around each carbon

What type of bond connects the two carbons in ethane?

A σ bond formed by the overlap of two sp³ orbitals

What orbitals form the C-H bonds in ethane?

sp³ orbitals from carbon overlapping with 1s orbitals from hydrogen

What orbitals combine to form sp² hybrid orbitals?

One s orbital and two p orbitals

Are there any remaining p orbitals in ethene?

Yes, one p-orbital remains unhybridised

How many sp² orbitals are formed in ethene?

Three

What geometry does sp² hybridisation produce?

Trigonal planar

What is the bond angle in ethene?

120°

How is the C=C double bond formed in ethene?

One σ bond formed in the head-on overlap of two sp³ orbitals

One π bond formed in the sideways overlap of two non-hybridised 2p orbitals

Why is a double bond shorter and stronger than a single bond?

Because it contains both a σ bond and a π bond

What orbitals combine to form sp hybrid orbitals?

One s and one p orbital

Are there any remaining p orbitals in acetylene?

Yes, two 2p orbitals remain unhybridised

How many sp orbitals are formed?

Two equivalent hybrid orbitals

What geometry results from sp hybridisation?

Linear

What is the bond angle in acetylene C₂H₂?

180°

How is a C≡C triple bond formed?

One σ bond and two π bonds from the two unhybridised p orbitals of each carbon

What hybridisation does oxygen use in water?

sp³

What is H-O-H bond angle in water?

105°

Why is the bond angle in water less than 109.5°?

Due to lone pair–bond pair repulsion

Can oxygen and nitrogen form double bonds?

Yes (e.g. carbonyl C=O and imine C=N)

What hybridisation do oxygen and nitrogen use in double bonds?

sp²

How is the π bond in carbonyl and imine groups formed?

By sideways overlap of unhybridised p orbitals

What defines a pi (π) bond?

Sideways overlap of parallel p orbitals