sp2 and sp Hybridization

Today we will discuss the formation of Boron trifluoride BF₃ molecule. Let’s write the ground state configuration of central atom B and F.

⁵B – 1s², 2s², 2p¹

⁹F – 1s², 2s², 2p⁵

B needs 3 unpaired electrons or you can say it needs 3 hybridised orbitals to make bonds with 3 atoms of F. That’s why B supplies some energy to 2s to lifts it to the level of 2p. In this way one of the paired electron of 2s jumps to the vacant orbital of 2p. Now this excited state has the following configuration:

⁵B – 1s², 2s¹, 2p²

Now B chooses one 2sorbital and two of the 2p orbital and hybridises them to get three hybridised sp²orbitals. These three hybridised sp²orbitals get arranged in a triangular shape (fan shaped). Now F atoms come closer to B so that their 2p_zorbitals get overlapped with these hybridised sp² orbitals and make bonds. Thus BF₃ molecule is formed by bonding between three sp²orbitals of B and p of 3 F atoms.

Let’s take another example of Beryllium difluoride BeF₂ molecule. Here you might raise a question about the nature of BeF₂ molecule. You might have debated that it is an ionic compound because Be belongs to the 2^nd group and F belongs to the 17^th group and they have a large electronegativity difference. I have told you in one of my previous posts that a bond will be purely covalent only if the electronegativity difference between bonded atoms is less than 1.7, but in BeF₂ it is 2.5 which is much greater and still BeF₂molecule is a pure covalent compound. It is one of the few exceptions but has an explanation. I will discuss it in detail in the coming post.

Write the ground state configuration of Be and F:

²Be – 1s², 2s²

⁹F – 1s², 2s², 2p⁵

Be has 2 paired electrons but it needs 2 unpaired electrons to combine with unpaired electrons of two atoms of F. Outer orbit of Be has two orbitals s and p so it promotes its one electron to the p orbital. Now it’s excited state configuration will be

²Be – 1s², 2s¹, 2p¹

With the promotion of electron, 2s orbital also gets promoted to the level of 2p orbital. Now Be picks ones and one p orbital and hybridises them to get two sphybridised orbitals. These two sphybridised orbitals get arranged in a linear shape.

Now two F atoms come closer to Be atom and sp hybridised orbitals of Be andp orbital of F get overlapped with each other and make bonds. Thus BeF₂molecule is formed by bonding between sp-p.

Up to now we have discussed those molecules which have single covalent bonds. In previous post of covalent compounds we have seen two more type of covalent bonds double bond and triple bond. How are these bonds formed? You know these bonds are formed by sharing of 4 and 6 valence electrons respectively. What is the role of orbitals in their formation? We will see the sights of orbitals in the formation of double and triple bond in the next post.

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