“Art has a way of confronting us, of reminding us, of engaging us, in what it means to be human, and what it means to be human is to be flawed, is to be contradictory, is to be often weak, and yet despite all of these what we would consider drawbacks, that we’re also quite beautiful. Spin is the opposite.” -Junot Diaz

“Two identical fermions cannot occupy the same state,” says the Pauli Exclusion Principle, one of the most fundamental laws of quantum physics that governs our Universe. But if that’s truly the case, then how can two hydrogen atoms bind together?

Image credit: New Scientist.

Image credit: New Scientist.

This deceptively simple question actually has a fascinating answer, and as it turns out, it holds the key to not only how they can bind together to form molecular hydrogen, but also to why you can bind no more than two hydrogens together!

Image credit: CERN, 2001, via http://www.physicsmasterclasses.org/exercises/keyhole/it/theory/main-5.html.

Image credit: CERN, 2001, via http://www.physicsmasterclasses.org/exercises/keyhole/it/theory/main-5.html.

Go and read the whole thing here.


  1. #1 Michael Kelsey
    SLAC National Accelerator Laboratory
    May 9, 2014

    @Ethan: Very nice! One editorial comment, and one question:

    In your last paragraph, I think you wanted to write “hydrogen molecule,” not “hydrogen atom.”

    The very nice set of 2D plots from New Scientist — do you happen to have a reference for the original paper? The figure itself just says (lower right margin) “SOURCE: PHYSICAL REVIEW LETTERS.” Well, duh 🙂

  2. #2 John Duffield
    May 10, 2014

    Michael: is this it?

    IMHO the thing to remember is that “electrons exist as standing waves” in atomic orbitals. And they don’t suddenly start existing as something else when they get ejected. That’s why you can diffract electrons.

  3. #3 Peter Krahulik
    May 12, 2014

    I like your spin explanation very much, but I want to ask the following: Would it be possible to collect a lot of hydrogen atoms exclusively with aligned spins (not bonding) to obtain atomic hydrogen gas? And what properties would it then have?

  4. #4 Anandaram Mandyam
    May 12, 2014

    The diagrams are lovely and fascinating.
    In this exposition devoted to the behavior of electrons in hydrogen, I thought you could have included some discussion on the other type of hydrogen atom known to exist in the photospheres of stars like the sun : the negative hydrogen ion in which both electrons stick to the same atom.
    Its discovery in the sun is a fascinating chapter!

  5. #5 Charles Herring
    June 11, 2014

    Thanks Bill and Ethan.

    If H2 is formed by symmetric and anti-sysmmetric H atoms does this mean there are 2 species of H2?

  6. #6 Sean T
    June 13, 2014

    Charles Herring,

    I think you may have misunderstood. A bond can only be formed if the electrons are in the spatially symmetric, spin anti-aligned state. That configuration results in a bonding orbital. The spatially anti-symmetric, spin aligned state produces an orbital in which there is no electron density overlap between the two nuclei, ie. an anti-bonding orbital.

    Basically, the spatially antisymmetric configuration is equivalent to two free hydrogen atoms and is less energetically stable than the bonding configuration. That is why we see diatomic hydrogen rather than free hydrogen atoms.

New comments have been disabled.