Formation of 2 pi bonds

1. Dec 19, 2004

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

Last edited: Dec 19, 2004
2. Dec 19, 2004

Gokul43201

Staff Emeritus
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.

3. Dec 19, 2004

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....

4. Dec 19, 2004

dextercioby

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...

5. Dec 19, 2004

Gokul43201

Staff Emeritus
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.

6. Dec 19, 2004

GCT

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