CBSE Class 11-science Answered

Dear Student

Ethene, C₂H₄

The simple view of the bonding in ethene

At a simple level, you will have drawn ethene showing two bonds between the carbon atoms. Each line in this diagram represents one pair of shared electrons.

An orbital view of the bonding in ethene

Ethene is built from hydrogen atoms (1s¹) and carbon atoms (1s²2s²2p_x¹2p_y¹).

The carbon atom doesn't have enough unpaired electrons to form the required number of bonds, so it needs to promote one of the 2s² pair into the empty 2p_z orbital. This is exactly the same as happens whenever carbon forms bonds - whatever else it ends up joined to.

Promotion of an electron

There is only a small energy gap between the 2s and 2p orbitals, and an electron is promoted from the 2s to the empty 2p to give 4 unpaired electrons. The extra energy released when these electrons are used for bonding more than compensates for the initial input.

The carbon atom is now said to be in an excited state.

When the carbon atoms hybridise their outer orbitals before forming bonds, this time they only hybridise three of the orbitals rather than all four. They use the 2s electron and two of the 2p electrons, but leave the other 2p electron unchanged.The new orbitals formed are calledsp² hybrids, because they are made by an s orbital and two p orbitals reorganising themselves. sp² orbitals look rather like sp³ orbitals that you have already come across in the bonding in methane, except that they are shorter and fatter. The three sp² hybrid orbitals arrange themselves as far apart as possible - which is at 120° to each other in a plane. The remaining p orbital is at right angles to them.

The two carbon atoms and four hydrogen atoms would look like this before they joined together:

We hope that clarifies your query.

Regards

Team

Topperlearning