Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How is hydrogen more stable in diatomic form?

  1. Oct 28, 2012 #1
    Hello Physicsforums,

    I'm curious to know what stability hydrogen receives through adding one more electron to its 1s sublevel level. The whole sublevel is at the same energy state, so why must it fill that one more space to attain stability? What does a full 1s orbital do to make it stable? I can see why it's inert, but we stay it's more stable.


  2. jcsd
  3. Oct 28, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Is molecular hydrogen said to be more stable than atomic hydrogen? Who by? And in what context?

    You need 13.6eV to break apart a H atom, but only 4.6eV to break apart an H2 molecule.

    The stability of a covalent bond is a QM effect from symmetry.
    Paired electrons in sub-shells sort-of "balance out the load" on an atom.
    But you should be aware that molecular energy levels are not the same as the atomic ones.

    See also:
  4. Oct 28, 2012 #3
    The reasoning here is the same as for all elements: the octet rule.
    The only difference is that hydrogen and helium are so simple that their "octet" is only two atoms.

    This stuff is really all about geometry. Everything in the universe is in a 3d matrix (well, 4d really, but we're being simplistic here). If there are electrons on opposite sides of a proton then everything is much more stable. Imagine... imagine a tight rope walker. When he's walking along the rope holding nothing, he's able to balance and move forward slowly. Give him a pole and he has some weight balancing him out horizontally, and he can move along a bit more steadily. Make him carry a weight in one hand though, and
    he's less stable.
    (Keep in mind that this is a completely different set of rules [kinetics], but I hope that it helps to illustrate the point a little.)
  5. Oct 28, 2012 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Though electrons in the ground state of hydrogen are symmetrically distributed ... but it's balanced here to pair spin-up and spin-down.
  6. Oct 29, 2012 #5


    User Avatar
    Science Advisor

    Yes, a H2 molecule is 4.6 eV more stable than 2 hydrogen atoms. However, the electrons are only seldomly both in the 1s orbital of the same atom. So it is quite misleading to speak of "filling up" the sublevel. Rather in H2 there is the possibility of concerted hopping of the electrons between the two atoms. The atomic wavefunctions are thus allowed to spread out in direction of the bond and at the same time reducing their extension in the direction perpendicular to the bond. This leads to an increased electrostatic attraction responsible for bonding.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook