[Rutherford]

I got a question related to the valence bond theory postulate of hybridization. If I take carbon for example, the configuration in excited state is: 1s² 2s¹ 2p_x¹ 2p_y¹ 2p_z¹. Now the 2s, 2p_x and 2p_y combine to make three sp² orbitals with equal energy content. The question is: are those hybrid orbitals lower in energy content than the 2p_z orbital that left?

[juanrga]

I got a question related to the valence bond theory postulate of hybridization. If I take carbon for example, the configuration in excited state is: 1s² 2s¹ 2p_x¹ 2p_y¹ 2p_z¹. Now the 2s, 2p_x and 2p_y combine to make three sp² orbitals with equal energy content. The question is: are those hybrid orbitals lower in energy content than the 2p_z orbital that left?

I will answer plus upload figure, but first let us know which you think is the answer and your reasoning.

[Rutherford]

I am not sure, but I will give it a shot: When hybridisation takes place, some energy is spent to make the hybrid orbitals and to rearrange the electrons (the energy spent is much less than the energy released when the hybrid orbitals make bonds). As energy was spent when they were made, I guess that they have a higher energy content than the p_z orbital that left. Good?

[juanrga]

I am not sure, but I will give it a shot: When hybridisation takes place, some energy is spent to make the hybrid orbitals and to rearrange the electrons (the energy spent is much less than the energy released when the hybrid orbitals make bonds). As energy was spent when they were made, I guess that they have a higher energy content than the p_z orbital that left. Good?

Let us see, when you hybridise, you are combining orbitals of different energy. In this case you are combining a 2s orbital with 2p orbitals. Combining orbitals of different energy you will come with hybrid orbitals whose energy will be in some place between the energies of the original orbitals {*}. Therefore you cannot obtain energies above the 2p when you combine it with an orbital 2s of lower energy. I have added a figure. It is about combining with 2p_y and 2p_z, but you only need to change the labels.


{*} This is like combining water at 100 ºC with water at 20 ºC, you obtain water with a final temperature between 20 and 100 ºC.

[Rutherford]

I made the mistake by looking at 2s as similar to 2p in energy content. Everything you wrote makes perfect sense to me and it is very clear. Thanks for the nice explanation.
On the diagram, the hybrid orbital energy content is closer to the 2p orbitals' content than the 2s orbital content, I guess that it is because as two of the p orbitals and only one of the s orbitals take part in the hybridisation. It is even closer because of the energy spent on hybridisation.