[Physics] Atomic orbitals and complex wavefunction

atomic-physicscomplex numbersorbitalsquantum mechanicswavefunction

I have read different questions related to the atomic orbitals labelled with 2px and 2py present here, such as What is the difference between real orbital & complex orbital? or Notation of complex valued atomic orbitals, but I've not found a complete clarification.

If $px$ and $py$ orbitals are a superposition of two state with definite $m$, which implies that the electron is partially in the $m=+1$ state and partially in the $m=−1$, why it's not so difficult to find books or slides where the $px$ is identified with the quantum number $m=1$ and the $py$ as $m=-1$ like in the image?

enter image description here

It seems that the wavefunction and its modulus squared says something that is similar to this:

enter image description here

So, where is the truth?
Why do we need a superposition for the $px$ and $py$ orbitals that doesn't comes out from spherical harmonics, and not for the $pz$?

I'm referring to the hydrogen wave functions:

$$\Psi_{n,l,m} (r, \theta, \phi) = N e^{\frac{-r}{n r_1}} R_n^l (r) P_l^m(cos \theta) e^{im\phi} \,.$$

Best Answer

The truth is your second image:

If you're going to use the magnetic quantum number $m$ as your index, then the $m=\pm 1$ wavefunctions look like rings. A wavefunction with well-defined $m=1$ or $m=-1$ (i.e. an eigenfunction of $\hat L_z$ with eigenvalue $1$ or $-1$) will never have the dumb-bell peanut-like shape of the $m=0$ orbital.


Images like the first one, where the $p_x$ and $p_y$ orbitals are identified as having $m=1$ or $m=-1$ are incorrect. Why are they relatively common? Because it's an easy mistake to make - but it doesn't stop it being a mistake.

What the cartesian $p$ orbitals do have is a well-defined $m^2=1$, which is to say, they are eigenstates of $\hat L_z^2$. However, that does not mean that they are eigenstates of $\hat L_z$, which they are not.

(They're also eigenstates of $\hat L_x$ and $\hat L_y$, but that's not a useful characterization - among other things, it does not generalize to higher $m$. If you want a full CSCO description, they're eigenstates of $L^2$, $L_z^2$, and the (commuting) parity operators $\Pi_x$ and $\Pi_y$.)


Now, just because they don't have a well-defined $m$, that doesn't mean that they're not useful, either, and for many applications it's more convenient (and perfectly legitimate) to have a real-valued basis set than to have full eigenvector relations for the basis as the $|l,m\rangle$ do (and indeed this is quite common practice in several applications). However, it's perfectly possible to use those orbitals without misrepresenting their relationship with the angular momentum, as your first image does.