Why two electrons per orbital?

Photo Credit: Rene Magritte - Decalcomania

Why two electrons per orbital?

Electron are negatively charged and repel each other.

How is it possible that within atoms and molecules they suddenly turn schizophrenic and two of them occupy the same orbital?

Frankly, this is nuts.

I have never encountered a simple, vivid and correct explanation which makes this contradiction acceptable. On the contrary, in basic chemistry, we simply assert that the electrons “fill-up” the orbitals that way with “opposite spin”.

But how does that explain anything?

What has spin got to do with any of this?

Electrons are four dimensional

The real answer is that electrons actually inhabit their own special four dimensional space.

This fourth dimension is special:

  • Unlike normal 3D space where \( x \), \( y \) and \( z \) may take any value in the range \(-\infty \) to \( +\infty \), the extra fourth dimension is bound to the surface a complex two dimensional circle.

  • The fourth coordinate is actually a kind of “wrapped up” line; and like any circle it is described by two complex numbers, say \( a \) and \( b \). These numbers are not independent but are connected by the equation \( |a|^2 + |b|^2 = 1\).

  • Indeed, since the extra dimension is a circle, one should actually think of it as comprised of two sub-dimensions. The numbers \( a \) and \( b \) are its coordinates along two abstract unit-vector directions which we usually call \( \ket{\uparrow} \) and \( \ket{\downarrow} \) (though probably should be called \( \ket{\uparrow} \) and \(\ket{\rightarrow}\)).

  • The spin coordinates \( a \) and \( b \) are analogous to the numbers \( x \), \( y \) and \(z\) describing distances along along the three dimensional unit-vector axes \( \ket{\B{e}\sub{x}}\), \(\ket{\B{e}\sub{y}}\) and \( \ket{\B{e}\sub{z}}\).

In a later post I will show that this extra fourth dimension is used to describe how the wavefunction of the electron changes when it is rotated in three dimensional space. For this reason, the extra dimension is called the electron spin. Because this fourth dimension is unique to electrons, it is often called an internal degree of freedom.

In contrast, the \( x \), \( y \) and \(z \) coordinates which are common to all particles are external degrees of freedom: these coordinates arise from translating the particle in space.

All this is a lot to take in; it is worth re-reading the previous paragraphs!

Time is not the fourth dimension!

To avoid any confusion, I should say that the fourth dimension we are talking about is not the time dimension; spin is a completely different additional dimension. Time is a plain old dimension like the other three spatial coordinates.

The spin of other particles

As a matter of fact, every fundamental particle has its own private wrapped up “spin” dimension attached to it – although the “size” of this dimension (two in the case of an electron) may be different for different particles.

  • For example, the photon has three spin components, two of which correspond to its helicity, used to describe its polarization.

  • Protons and neutrons have an extra fourth dimension with two sub-dimensions, exactly like electrons.

In any case the extra private wrapped-up dimension describes how the particle behaves when it rotates, and so the extra dimension is always called spin.

Why are spin coordinates so different to position coordinates?

Why do all particles have the same \( x \), \( y \) and \( z \) coordinates to describe translation, but different coordinates to describe how they rotate?

Another way to ask this is: why are the external degrees of freedom always the same for every particle, but the internal degrees of freedom may be different?

The answer is that it just happens translations are easier to describe mathematically than rotations. There is only one way to describe translations for a wavefunction, and that is with three corrdinates. By contrast, since rotations are more complicated (I hope you agree!) the way an object can behave under rotations can possibly display more variation.

Personally I think the distinction between internal and external degrees of freedom is not an important one. In either case, the electrons “coordinates”, whatever they are, are used to describe how its wavefunction changes when you translate or rotate it.

Why can’t we “see” spin like position?

When you look at things, your eyes register different positions when an object has different \( x \), \(y \) and \( z\) coordinates when photons of arbitrary polarization scatters from that particle into your eyes.

If sources of light were generally available which could detect helically polarized light, and if our eyes were adapted to such polarized light, we might actually be able to “see” a particle differently according to what spin it has.

Sadly, our eyes do not have this ability.

What luck that we have instruments to probe things where our eyes fail us!

Why the extra dimension makes sense

Nevertheless, it is not immediately obvious that electrons have their own extra fourth dimension.

The evidence for it came from atomic electronic structure.

From an elementary perspective it seems as if the electrons just occupy the same orbital as they “fill up”. But it only looks like there are two electrons per orbital.

In actual fact, each electron is in its own four dimensional spinorbital.

A spinorbital is just a normal orbital three dimensional orbital (wavefunction) extended into the fourth dimension.

Now two key points:

  1. The spinorbital notion allows to explain why two electrons occupy an orbital because an orbital is a 3D concept. It looks to us as if they are in the same orbital, but in fact they are in different spinorbitals. They are actually in different 4D “positions”.

  2. This spinorbital idea also allows to explain why two electrons would even want to be in the same 3D orbital. The reason is that in this way they can both get close to the positively charged nucleus in 3D to which they are attracted. However, at the same time they are away from each other in the fourth dimension if they are in different spinorbitals. (It is worth noting that the electrical repulsion between the electrons is not reduced because the electrons have a different fourth dimension, since repulsion operates only in 3D)

We represent this different location in the fourth direction by an up- or down-arrow corresponding to the fact that they are (more or less) on different axes the spin circle. But you already know about the up- and down- arrows.

So the answer to the riddle “why do electrons occupy the same orbital” is that “they don’t”. They stay away from each other, while still maintaining their mutually close distance (in 3D space) to the positively charged nucleus.

Without invoking the extra dimension, it is very hard to reconcile why electrons should actually occupy the same region of space. In fact, it took Pauli quite some time to come to this conclusion himself.

Antisymmetry and electron spin

Previously we derived the fact that the wavefunction for identical particles either reverses sign or stays the same. If the former, the particle was called fermion if the latter, a boson.

For fermions, the change is sign or antisymmetry was called the Pauli principle.

Now: rotating two electrons around their common midpoint is the same as swapping their positions. Swapping is more complicated than rotating when there are more than two electrons.

Nevertheless, using the case of just two electrons, one might expect that there is a connection between a particles spin (i.e. how the wavefunction behaves when rotated) and whether it is a fermion or boson (i.e. how the wavefunction behaves when the coordinates are swapped).

This is in fact the case.

Pauli received the Nobel Prize for proving that, if a particle has a spin quantum number which is a half integer, a wavefunction involving two of those particles must reverse sign whenever the coordinates of those particles are swapped. This is known as the spin-statistics theorem and it has to be one of the deep facts of the universe. T

Unfortunately, the proof of this theorem by Pauli is rather complicated and it involves ideas from relativistic quantum mechanics and field theory.

I think the general view is that Pauli has not actually proved this theorem but merely shown that it follows from other sensible assumptions made in quantum mechanics. This is a rather unsatisfactory state of affairs, and Richard Feynman said so.

Perhaps you can find an alternative, simple explanation?

Matter-like particles

In fact, all matter-like particles must obey the antisymmetry principle.

That is because the antisymmetry principle demands particles have a zero probability of occupying the same region of space, exactly as one would expect for “solid” particles. Such solid particle simply can’t occupy the same space at any time (remembering that by “space” we include the particles internal fourth dimension).

By contrast, it seems we can cross as many laser beams as we like, proving that the same does not hold for particles of light. We conclude that light particles must be bosons and that bosons are not matter-like.

A vivid example

image image
Untitled, Krisar (2013); Les Liasons Dangerous, Magritte (1935)

All of this is rather abstract.

In the hopes of making the essential idea clearer in class, I have tried the following demonstration.

  • I ask two boys to stand up in the same rows of the lecture theatre seats near the front – where the seats are never occupied – and move towards each other. Of course, they are unable to pass each other. These two boys are supposed to represent two electrons of the same spin coordinate. Since they have the same spin coordinate they cannot “pass” each other (they are trapped between the rows).

  • Next I ask a boy and a girl in different but consecutive rows of seats to move towards each other. When they are about to cross I shout out “stop”. At this point, the rest of the students (who are clustered at the back of the theatre) can see that the girl and the boy are roughly “on top” of each other. They look as if they are occupying the same space, but are actually on different rows. This is exactly analogous to the case of two electrons with different spin being able to cohabit the same three dimensional position.