14.1 & 14.2 - Sigma, Pi bonds & Hybridisation

Sigma bonds

* Single covalent bonds are sigma
* These are formed by the **‘end on’** overlap of two atomic or hybrid orbitals
* The orbitals are distributed symmetrically around a line joining the two atoms

Pi bonds

* When a double covalent bond forms, it is impossible for 2 sigma bonds to form
* One sigma bonds forms and there is a **‘side-ways’** overlap of two non-hybridised atomic p-orbitals
* The overlap occurs above and below the sigma bond
* Accounts for the shorter length of a double bond, and also a restriction in its ability to rotate
* In a triple covalent bond, there are **1 sigma and 2 pi bonds** formed by 2 ‘p orbitals’ on each of the atoms that overlap

Essential idea of hybridisation

Results form the mixing of atomic orbitals to form the same number of new equivalent hybrid orbitals that can have the same mean energy as teh contributing atomic orbitals

Hybridisation

* Is a theory to bring together the shapes of atomic orbitals
* **The combining of atomic orbitals to produce new hybrid bonding orbitals**
* Atoms that covalently bond do so by overlapping their orbitals to share electrons

Problems in hybridisation

* Some of the electrons to be shared are in
* Different types of orbitals (s,p,d,f)
* They often have different shapes (spherical or dumbbell)

Solution

* Atoms overcome this problem by putting its bonding electrons into bonding orbitals (hybrid orbitals)
* This bonding orbital is a combination of the types, shapes, and energies of the s,p,d,f orbitals involved and hence is called a hybrid orbital
* Process is called hybridisation

Types of hybridisation

* The type of hybridisation depends on the number of electron domains around the central atoms
* Any lone pairs of electrons which are in the outer orbital of an atom but are not being used in bonding are still put into bonding orbitals to keep them as far away from the bonded electron pairs
* When an atoms forms a **double** or a **triple** bond with another atoms, it only used **one bonding orbital** to form a **sigma bond**
* Its other electrons are kept in its atomic p-orbitals where they can be used to form one or more pi bonds

Sp3 hybridisation

* Occurs in atoms which form **4 electron domains**
* The hybrid orbitals are arranged in a **tetrahedron**
* E.g. Carbon in CH4 and diamond

Sp2 hybridisation

* Occurs in atoms which have **3 electron domains**
* The hybrid orbitals are arranged in a **Trigonal planar** arrangement
* E.g. BF3, C in graphite and in ethene

Sp hybridisation

* Occurs in atoms which have **2 electron domains**
* These orbitals are arranged in a **linear** arrangement
* E.g. BeCl2 ; CO2
* Thye type of hybrid orbitals for the **Trigonal bipyramid** and **octahedral** shapes involves **s,p,d,f** hybrids