Chemical - Bonding-7

1. Formation of boron trifluoride (BF₃). Boron (₅B) atom has ground state configuration as 1s² 2s², 2p¹. But in the excited state its configuration is 1s², 2s¹, 2p_x¹, 2p_y¹. One 2s-orbital of boron intermixes with two 2p-orbitals of excited boron atom to form three sp² hybrid orbitals as shown in figure below.

The sp² hybrid orbitals of boron are directed towards the corners of equilateral triangle and lie in a plane. Each of the sp² hybrid orbitals of boron overlaps axially with half-filled orbital of fluorine atom to form three B-F sigma bonds as shown in figure below.

Becasue of sp² hybridisation of boron, BF₃ molecule has triangular planar shape.

2. Formation of ethylene (C₂H₄). Both the carbon atoms in ethylene assume sp² hybrid state. In acquiring sp² hybrid state, one 2s-orbital and two 2p-orbitals of excited carbon atom get hybridised to form three sp² hybridised orbitals. However, one orbital of 2p-sub-shell of the excited carbon atom does not take part in hybridisation. The promotion of electron and hybridisation in carbon atom is shown in figure below.

As already indicated, the three sp² hybrid orbitals lie in one plane and are oriented by space at an angle of 120° to one another. The unhybridised 2p-orbital is perpendicular to the plane of sp² hybrid orbitals as shown in figure below.

In the formation of ethylene, one of the sp² hybrid orbital of carbon atom overlaps axially with similar orbital of the other carbon atom to form C—C sigma bond. The other two sp² hybrid orbitals of each carbon atom are utilised for forming sp²-s sigma bond with two hydrogen atoms.

The unhybridised p-orbitals of the two carbon atoms overlap sidewise each other to form two p clouds distributed above and below the plane of carbon and hydrogen atoms figure below.

Thus, in ethylene, the six atoms (bonded by sigma bonds) lie in one plane while the p bond is projected perpendicular to the plane of six atoms (two C atoms and four H atoms).

In ethylene molecule, the

C = C bond consists of one sp²-sp² sigma bond and one p bond. Its bond length is 134 pm. C—H bond is sp²-s sigma bond with bond length 108 pm. The H—C—H angle is 117.5° while H—C—C angle is 121°.

(iii) sp-hybridisation. This type of hybridisation involves the mixing of one orbital of s-sub-level and one orbital of p-sub-level of the valence shell of the atom to form two sp-hybridised orbitals of equivalent shapes and energies. These sp-hybridised orbitals are oriented in space at an angle of 180° figure below. This hybridisation is also called diagonal hybridisation. Each sp hybrid orbital has equal s and p character, i.e., 50% s-character and 50% p-character. The molecules in which the central atom is sp-hybridised and is linked to two other atoms directly have linear shape.

Let us study some examples of molecules involving sp hybridisation.

1. Formation of beryllium fluoride (BeF₂). Beryllium (4Be) atom has a ground state configuration as 1s², 2s². In the excited state one of the 2s-electron is promoted to 2p-orbitals. One 2s-orbital and one 2p-orbitals of excited beryllium atom undergo hybridisation to form two sp-hybridised orbitals as described in figure below.

The two sp hybrid orbitals are linear and oriented in opposite directions at an angle of 180°. Each of the sp-hybridised orbital overlaps axially with half-filled orbital of fluorine atom to from two Be—F sigma bonds figure below.

Due to the sp-hybridised state of beryllium, BeF₂ molecule has linear shape.

2. Formation of acetylene (CH CH). Both the carbon atoms in acetylene assume sp-hybrid state. In acquiring sp-hybrid state, one 2s orbital and one 2p-orbital of excited carbon atom (1s² 2s¹ 2p_x¹ 2p_y¹> 2pz1) get hybridised to form two sp-hybridised orbitals figure below.

The two sp-hybrid orbitals of carbon atom are linear and are directed at an angle of 180° whereas the unhybridised p-orbitals are perpendicular to sp-hybrid orbitals and also perpendicular to each other as shown in figure below.

In the formation of acetylene, carbon atom uses its one of the sp-hybrid orbital for overlapping with similar orbital of the other carbon to form C—C sigma bond. The other sp-hybrid orbital of each C atom overlaps axially with 1s-orbital of H atom to form C—H sigma bond. Each of the two unhybridised orbitals of both the carbon atoms overlap sidewise to form two bonds. The electron clouds of one bond lie above and below the internuclear axis whereas those of the other bond lie in front and back of the inter-nuclear axis.

The overlapping of orbitals has been shown in figure below.

The four clouds so formed further merge into one another to form a single cylindrical electron cloud around the internuclear axis representing C—C sigma bond. It has been shown in figure below.

Gigure. Orbital diagram of BeF₂.

Thus, in acetylene molecule,

C C bond consists of one sp-sp s bond alongwith two p bonds. The CC bond length is 120 pm. C—H bond is sp-s sigma bond. The H—C—C angle is 180°, i.e., the molecule is linear.