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The electronic configuration of beryllium atom is 1s2, 2s2. According to the valence-bond theory, beryllium is expected to be an inert atom, as it does not possess any unpaired electron. However, the beryllium atom forms compounds with valency equal to two. In order to explain this bivalence, one electron from the 2s orbital may be promoted to a vacant 2p orbital. The energy required for this purpose may be considered to be recoverable from the bonding which beryllium atom enters with the other atoms. Now the beryllium atom contains two unpaired electrons; one in the non-directional 2s orbital and the other in the directional 2p orbital. So, the atom of beryllium is expected to form two different types of bonds. But in actual practice, both the bonds formed by this atom are found to be equivalent with bond angle equal to 180o. In order to explain this fact, the concept of hybridization is introduced.

The process of hybridization involves the mixing of two or more than two atomic orbitals centered on the same atom to give equivalent number of completely identical degenerate orbitals (i.e. all orbitals have the same shape, size and energy). These orbitals are termed as hybrid orbitals. The process of hybridization does not involve any gain or loss of energy, i.e. the sum of the energies of hybrid orbitals is equal to the sum of the energies of orbitals, which are being mixed to get hybrid orbitals.

A brief account of the kind of hybridization commonly met with is described as below:

In sp hybridization, one s orbital and one p orbital belonging to the valence shell of an atom both mix to provide two degenerate sp orbitals. Each orbital includes fifty per cent contribution from each of s and p orbitals. Unlike 2p orbital, sp hybrid orbital has one highly enlarged lobe and one highly shortened lobe.

 The Schematic Formation of sp Hybrid Orbitals

Example 1
Beryllium Hydride

Hybridization Scheme

Electronic configuration of 4Be : 1s22s2

Electronic configuration of 1H : 1s1

The molecule of BeH2 is found to be linear indicating that Be in BeH2 is sp hybridized. The electronic configuration of Be in the free state and that existing in the BeH2 molecule are as follows:

   Be atom in free state                        Be atom in BeH2   

Each Be—H bond is formed by the overlapping of sp(Be) and 1s(H) orbitals.


 Bonding in BeH2
Example 2  
Hybridization Scheme Lewis structure of ethyne H : C C : H. The molecule of ethyne is linear with bond angle 180°. This indicates that the carbon atoms in ethyne are sp hybridized. The electronic configuration of carbon atoms and the one existing in ethyne are as follows:
The two sp hybrid orbitals are oriented at an angle of 180° with respect to each other. The two p orbitals are perpendicular to each other and also to sp orbitals.

One of the two sp orbitals of one carbon atom is involved in the σ bonding with hydrogen atom and the other is involved in σ bonding with the second carbon atom. The two 2p orbitals make two π bonding with the second carbon atom.

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