Lecture 10

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Learning Outcomes At the end of the lesson the students should be able to: 1. Explain how orbital overlap produces a covalent bond. 2. Draw and describe the formation of sigma(σ) and pi(π) bonds. 2

Valence bond theory Valence bond theory is based on the following four postulates: Postulate 1: A bond is formed when atomic valence orbitals overlap with each other.

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Postulate 2: Overlapping orbitals contain a pair of electrons. Postulate 3: Electron density concentrates between bonded atoms.

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Postulate 4: The strength of the bond depends on the degree of overlapping.

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 Covalent bond is formed when atomic orbitals overlap.  Only 2 electrons with their spins paired may be shared in one set of overlapping orbitals.  2 types of bond: • sigma bond (σ) • pi bond (π)

The simplest molecule we can discuss in terms of valence bond theory is a hydrogen gas, H2

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- Sigma bond occurs when there is one bonding interaction that results from the overlapping of two orbitals end to end.

•Result from the overlapping of two s orbitals. Example: H2 H

:

↑ 1s

• Result from the overlapping of s and p orbitals. Example: HF

H

:

F

:

↑ 1s ↑↓

↑↓ ↑↓ ↑_

2s 2p valence electrons •One of the 2p orbital of F atom is occupied by a single electron. •H electron and the F electron pair up and be shared between the two nuclei.(s orbital of H and one p orbital of F will overlap to share their electrons. 11

iii) Result from overlapping of two p orbitals. Example: Cl2 ↑

Cl :

↑↓ 1s

↑↓ 2s

↑↓ ↑↓ ↑↓ 2p

↑↓ 3s

↑↓ ↑↓ 3p

valence electrons

- One of the 3p orbitals is occupied by a single e

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Example 2: F2

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 Two lobes of p orbitals overlap side-to-side gives pi bond (sideways overlapping of two p orbitals).  It occurs in molecules with double or triple bonds π

Example: N2

Example 2:

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Exercise 1: Give the number of σ and π bonds in: a) N2 molecule b) O2 molecule c) F2 molecule

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Conclusion The formation of a sigma bond: 1. Formed by overlapping of s orbitals 2. Formed by overlapping of s and p orbitals. 3. Formed by head-to-head overlapping of p orbitals. The formation of a pi bond: 1. Formed by side-to-side overlapping of p orbitals. 22

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