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Our chemistry teacher said that two $\mathrm{p}_z$ orbital only form sigma bond whereas two $\mathrm{p}_x$ and two $\mathrm{p}_y$ orbitals form only pi bond.

He also stated that only $\mathrm{p}_z$ orbital forms a sigma bond with the $\mathrm{s}$ orbital.

But in our book it is written that $\mathrm{p}_x$, $\mathrm{p}_y$, and $\mathrm{p}_z$ orbitals can form both sigma and pi bonds.

Please give me the correct explanation.

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    $\begingroup$ See, the three p orbitals look exactly the same. How do you know which of them is px and which is pz? $\endgroup$
    – Ivan Neretin
    Jun 25, 2019 at 15:45
  • $\begingroup$ @Ivan Neretin Ok so does that mean that any p orbital can form a sigma or pi bond. $\endgroup$
    – Ashok Sharma
    Jun 25, 2019 at 16:27
  • $\begingroup$ Yes and no. Some people (including you teacher) use a different convention. $\endgroup$
    – Ivan Neretin
    Jun 25, 2019 at 17:00

1 Answer 1

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It's a question of

(1) labeling conventions (what you call x, y and z in the coordinate system defining the position of two bonded atoms)

and

(2) the orientation of the two overlapping orbitals from the bonding atoms

If an overlap maximum occurs on the internuclear axis between the atoms, you call the resulting bonding orbital a "sigma" orbital. If maximum overlap occurs on an axis parallel to the internuclear axis, you call this type of bonding "pi". Either s or p orbitals can form sigma bonding orbitals, but s orbitals generally form sigma orbitals, whereas of the 3 conventional p orbitals, only one has lobes oriented along the internuclear axis (this orbital is usually called $p_z$ because its principal symmetry axis coincides with the internuclear axis, which is also labeled the z axis) and forms sigma bonding orbitals. The remaining two (x and y) form pi orbitals. This is illustrated schematically in the following figure:

enter image description here

(note the AOs don't overlap significantly, focus is on showing their relative orientation and labels)

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  • $\begingroup$ I appreciate to explain the following paragraph by using a picture "only one has lobes oriented along the internuclear axis (usually called $p_z$ because its principal symmetry axis coincides with the internuclear axis which is also labeled the z axis) and forms pi bonding orbitals. " $\endgroup$
    – Adnan AL-Amleh
    Jun 27, 2019 at 4:00
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    $\begingroup$ @AdnanAL-Amleh Thanks for bringing the garbled sentence to my attention. Also, I added a figure. $\endgroup$
    – Buck Thorn ♦
    Jun 27, 2019 at 9:49
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    $\begingroup$ @Kaushik There is a lot of freedom in deciding what you call the "z" coordinate, but usually the convention is that x and y are related by a rotation symmetry whereas z is unique and not related to x or y by a rotation. $\endgroup$
    – Buck Thorn ♦
    Sep 26, 2019 at 17:28
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    $\begingroup$ Sigma bonds usually form first (lower in energy), so in this case pz would be filled first. But I'd hate to generalize without knowing what particular problem you are referring to. $\endgroup$
    – Buck Thorn ♦
    Sep 26, 2019 at 17:29
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    $\begingroup$ @Kaushik The orientation of the orbitals between the atoms is more important then what you call them (x,y or z). If x lies in the internuclear axis, then px is involved in sigma bonds. $\endgroup$
    – Buck Thorn ♦
    Sep 26, 2019 at 17:39

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