Molecular Orbits....what exactly is meant by orbitals having phase?

[CuriousBanker]

Hello. I understand constructive/destructive interference for waves in general. But why do orbitals have phase? What is meant by phase for an orbital? What is the phase describing? Isn't the wave function describing where you are likely to locate the electron? Why is one positive and one negative (or one red and one blue)? I don't get what it is, or how two atoms forming a molecule and can have both constructive and destructive interference at the same time

 

[Drakkith]

Why is one positive and one negative (or one red and one blue)?

The wavefunction can yield either a positive or negative value I believe, but it is the value squared that determines the probability. Hence you can have a negative value for the wavefunction and still get a positive result for the probability.

 

[Drakkith]

I don't get what it is, or how two atoms forming a molecule and can have both constructive and destructive interference at the same time

Unfortunately the wave function is not a physical wave like you can see on the ocean, nor is it a sound wave which you can feel, nor is it an electromagnetic wave which can be picked up by antennas. It is much more abstract. The best way I can describe it (and others will probably be able to describe it much better since I don't understand it that well) is that nature behaves in a way which can only be described by using math that is very similar to what describes classical waves. So we call the equation which governs this behavior a wave function.

What exactly is 'waving' here is, well, nothing. That's part of the abstractness and is one of the things that makes quantum physics so non-intuitive to newcomers.

 

[CuriousBanker]

I can get behind that idea...I'm guessing when I get to quantum I'll understand it better? I'm still years away lol

 

[Dr. Courtney]

What exactly is 'waving' here is, well, nothing. That's part of the abstractness and is one of the things that makes quantum physics so non-intuitive to newcomers.

I would not phrase it that way. Orbitals are true matter waves. Like other waves, a key feature is that matter waves, including orbitals, will interfere with other waves of the same type. If this were not so, devices like the atom interferometer would not be possible. See:
https://en.wikipedia.org/wiki/Atom_interferometer
If two matter waves are "out of phase" then destructive interference occurs and the matter in question has a much lower probability of being found in the location of destructive interference. If two matter waves are "in phase" then constructive interference occurs and the matter in question has a higher probability of being found in the location of constructive interference. This is analogous to how a photon is more likely to be found when electromagnetic waves are in phase.

As with most waves, the interference concept and implications of phase are easier to visualize when considering traveling waves than standing waves. Orbitals are standing waves, which makes phase harder to visualize. Orbitals are further complicated compared with most other waves, because the wavelength is changing along most paths over which one would consider questions of amplitude and phase. But these complications don't really change the meaning of "phase" in the context of orbitals. One can think of the orbitals that exist as the ones where following the wave over allowed periodic classical paths produces constructive interference.

 

[Drakkith]

I would not phrase it that way. Orbitals are true matter waves. Like other waves, a key feature is that matter waves, including orbitals, will interfere with other waves of the same type. If this were not so, devices like the atom interferometer would not be possible.

Ah, but there's still the question, "what is waving?"