Understanding the Hybridization and Formation of Pi Bonds in Ammonia and Ethyne

[Clari]

In ammonia, why the N atom is sp3 hybridized and not sp2 hybridized...since there are only 3 bondings...

Also, in ethyne, that is H-C(triple bond)C-H...for the formation of 2 pi bonds, why it is the unhybridized 2px orbital and 2py orbital of each C atom overlap with each other? How about the 2pz orbital?


any help would be appreciated

 

[Gokul43201]

In NH3, there are 4 electron pairs about the N atom - you're omitting the lone pair.

The triple bond in $H-C \equiv C-H$ is basically a sigma bond using one of the two 2s electrons (the other 2s electron is used for the sigma bond with H), and a pair of pi bonds using both of the 2p electrons. Since C has only 2 electrons in the 2p subshell, and since px and py are lower energy orbitals than pz, the pi bonds must involve an overlap of only these orbitals.

 

[Clari]

Gokul43201:

but in my textbook, it is said that orbitals in the same subshell is at the same energy level...I am confused now...

 

[dextercioby]

Gokul43201:

but in my textbook, it is said that orbitals in the same subshell is at the same energy level...I am confused now...

For isolated Carbon atoms they do:$2p_{x},2p_{y} & 2p_{z}$ have all the same energy.But when the Carbon atom reacts with nonmetalic elements,like Hydrogen,those orbitals do not behave in the same manner.That's because 2 of them have one electron and the remaining one ($2p_{z}$) has none.The 2 orbitals with one electron they "hybridize" with the $2s$ orbital which has 2 electrons,resulting in 4 hybridized orbitals of smaller energy than the unhybridized $2p_{z}$.Since it overlaps 3 hybrid orbitals (one $\sigma$ and 2 $\pi$) with another C atom,it has a triple bond.Since 2 of the bonds are $\pi$,and another 2 are $\sigma$,it id hybridized $sp$.


Daniel.

PS.Maybe Gokul can put it in a more coherent way...

 

[Gokul43201]

As Dexter mentioned, your textbook is right only when it talks about isolated atoms.

However, when a molecule is being formed, the direction along the line joining the atoms breaks the degeneracy (x, y and z are not symmetric anymore; the z direction is the internuclear direction and so, is different from x and y) between px, py and pz.

 

[GCT]

A "pz" pi molecular orbital would be along the same axis as the sigma, perhaps this clarifies things a bit more.