[Uri]

In molecular orbital theory, what is an non-bonding orbital?

[Kryolith]

If atomic orbitals don't have the appropriate symmetry for an interaction, nonbonding molecule orbitals result. For example the t2g orbitals in an octahedral complex.

[irishnenglish]

Or if you are 16 like me

lone pairs are non bonding pairs

found on pyramidal structures like NH3

non bonding pairs repel more than bonding pairs and so there is a smaller angle between the hydrogens.

[Uri]

If atomic orbitals don't have the appropriate symmetry for an interaction, nonbonding molecule orbitals result. For example the t2g orbitals in an octahedral complex.

But what is an nonbonding orbital? What do you mean by the appropiate symmetry for an interaction?

P.S Where is the MO diagram taken from?

[Kryolith]

Orbitals with the same symmetry form bonding and antibonding orbitals. Orbitals with a different symmetry can not interact, hence they are nonbonding orbitals. Appropriate was the wrong word, sorry.

I found this scheme with google's picture search, but I don't know what I searched anymore.

[Uri]

Orbitals with the same symmetry form bonding and antibonding orbitals. Orbitals with a different symmetry can not interact, hence they are nonbonding orbitals. Appropriate was the wrong word, sorry.

I found this scheme with google's picture search, but I don't know what I searched anymore.

You mean molecular symmetry? Why can't orbitals with a different symmetry interact?

[Kryolith]

metaphorically speaking they cannot overlap if the symmetry is not the same.

[Alpha-Omega]

By definition, molecular orbitals lower in energy than the atomic orbitals from which they are derived
are called bonding molecular orbitals; those with the same energy are called non-bonding molecular
orbitals and those higher in energy are called antibonding molecular orbitals.

A non-bonding orbital is one that is isolated because it has nothing to overlap or match with.

What happens depends on whether the symmetry is allowed or forbidden.

For Example look at something simple like HF

There is a little bit of mixing between the H 1s and the F 2sorbital but it interacts mostly with
the 2pz.

F also has the 2px and 2py orbitals that cannot interact with the H 1s orbital because they
have the wrong symmetry!

If you try to combine these orbitals with the 1s on H, you will find that the overlap integral, S, is equal to 0.  So  these orbitals are exclusively found on the F atom and are called non-bonding. The energies of these orbitals do not change from the energies in the F atom.

The combinations of sigma symmetry: (note that the 1s orbital on H is closer in energy to the 2pz orbital on F so we will look at that combination because there will be more interaction)

The orbitals that are derived mostly from F are going to be closer to the energies of the atomic orbitals of F and vice versa.

See Attachent:

[Uri]

metaphorically speaking they cannot overlap if the symmetry is not the same.

What do you mean by overlap?

By definition, molecular orbitals lower in energy than the atomic orbitals from which they are derived are called bonding molecular orbitals; those with the same energy are called non-bonding molecular orbitals and those higher in energy are called antibonding molecular orbitals.

What do you mean by energy? You mean chemical energy?

A non-bonding orbital is one that is isolated because it has nothing to overlap or match with.

What do you mean by overlap or match with?

What happens depends on whether the symmetry is allowed or forbidden.

By symmetry you mean molecular symmetry?

[Kryolith]

I attached a picture, I've drawn to demonstrate the three basic interactions. It is just a (simple) example. black/black or white/white is a bonding interaction, white/black or black/white an antibonding one. A nonbonding interaction is the result of the sum of an antibonding + bonding interaction (==> summa sumarum no interaction)

A bonding interaction decreases the potential energy of the system/the orbitals. An antibonding interaction increases the potential energy.

Speaking about the overlap of orbitals is a figurative description of quantum mechanics. Very simple explanation: Try to imagine that two orbitals approach. If they are near enough they "overlap". This will result in

*a stabilization (black/black for example), which means that the potential energy of the newly formed (bonding) molecule orbital is lowered in comparison to the atomic orbitals

and

*a destabilization (black/white for example), which means that the potential energy of the newly formed
(antibonding) molecule orbital is increased in comparison to the atomic orbitals.

[Uri]

I attached a picture, I've drawn to demonstrate the three basic interactions. It is just a (simple) example. black/black or white/white is a bonding interaction, white/black or black/white an antibonding one. A nonbonding interaction is the result of the sum of an antibonding + bonding interaction (==> summa sumarum no interaction)

A bonding interaction decreases the potential energy of the system/the orbitals. An antibonding interaction increases the potential energy.

Speaking about the overlap of orbitals is a figurative description of quantum mechanics. Very simple explanation: Try to imagine that two orbitals approach. If they are near enough they "overlap". This will result in

*a stabilization (black/black for example), which means that the potential energy of the newly formed (bonding) molecule orbital is lowered in comparison to the atomic orbitals

and

*a destabilization (black/white for example), which means that the potential energy of the newly formed
(antibonding) molecule orbital is increased in comparison to the atomic orbitals.

The picture you've drawn, does it have a name? How is it called? What do you mean by stabilization and destabilization?

[Yggdrasil]

Remember that each orbital represents a wavefunction.  Like waves, wavefunctions can add either constructively or destructively.  In the diagram shown by Kryolith, lets assume that the white represents areas where the wavefunction is positive and the black represents areas where the wavefunction is negative.

In the first diagram, the positive end of the p-orbital interacts with the negative end of the other p-orbital.  Since the two wavefunctions have opposite signs in the area where they overlap, the combination of these two orbitals results in destructive interference.   This decreases the likelihood that electrons will be found between the nuclei.  Since the presence of negatively charged electrons between the two positive nuclei helps to stabilize chemical bonds, removing electrons from the internuclear space destabilizes the chemical bond by increasing the electrostatic repulsion between the two nuclei.

In the second diagram, the two negative ends of the p-orbital overlap.  This causes constructive interference because the wavefunctions have the same sign in the area where they overlap.  This constructive interference increases the likelihood that electrons are found in the internulcear space and makes the chemical bond more stable.