Why do atoms bond?

  1. Can someone post a picture of 2 bonded atoms... But not those googled ones.

    I am trying to understand how does that look at molecular level. Why would 2 atoms bond to form a molecule. eg. hydrogen, covalent bond.

    I know now 2 atoms of hydrogen have full shell, but that electron doesn't belong to that hydrogen, it doesn't orbit around nucleus of one hydrogen, it is still orbiting around the other one.
    What good does one hydrogen have from that?

  2. jcsd
  3. fzero

    fzero 2,601
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    A stable bond has the property that the bound state is a lower energy configuration than the unbound state. A physical system will always seek the configuration that minimizes the potential energy.

    In a covalent bond, there is at least one electron which is shared by both nuclei. This is a valence electron and "covalence" refers to the fact that these electrons are shared.
  4. DrDu

    DrDu 4,094
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    The basic reason was elucidated first by H. Hellmann in the 1930's and has much been discussed since then. The electron is a quantum mechanical object, and it fulfills the position momentum uncertainty relation, namely the product of momentum uncertainty and spatial uncertainty is greater than Plancks constant.
    By forming a bond, the spatial uncertainty of the electronic wavefunction increases as the electron is not confined to the realm of one atom but only to that of two. Hence the momentum uncertainty becomes lower. Howewer, the square of the momentum uncertainty is basically the kinetic energy of the electron which hence gets lower, too.
    For more info, see the article by
    Kutzelnigg, W. (1973), The Physical Mechanism of the Chemical Bond. Angewandte Chemie International Edition in English, 12: 546–562. doi: 10.1002/anie.197305461
  5. So basically, reason for bonding is lowering the energy?

    Last orbit has high energy, and when bonded has lower? Makes sense.

    But how does this look, this bond? Do electrons go around both nuclei or something else?
  6. DrDu

    DrDu 4,094
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    Yes, the energy of two bonded atoms is lower than the energy of two separate atoms. That's the definition of a bond.
    How it looks is a more difficult question as it is an essentially quantum mechanical effect.
    But it's certainly true that the electrons ramble around both atoms.
  7. Claude Bile

    Claude Bile 1,479
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    When you bring two identical atoms together, the energy levels of each individual atom split into bonding and anti-bonding levels; so called because the bonding orbital has an energy which is lower than the sum of the original energy levels (thus contributing to the atoms bonding), while the anti-bonding orbital has an energy which is higher (thus contributing against bonding).

    Generally electrons will occupy lower (bonding) energy levels first, which is why molecules such as O2 and H2 are prevalent, while for noble gases, where bonding and anti-bonding orbitals are occupied in equal numbers (due to the full valence shells), the atoms appear in isolation.

  8. A. Neumaier

    A. Neumaier 1,949
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    Instead of thinking of electrons as orbiting the atoms, it is more profitable to think of them in terms of the electron field they form - a sort of fluid flowing around the nuclei and filling the space in between.

    See Chapter A6 of my theoretical physics FAQ at http://www.mat.univie.ac.at/~neum/physfaq/physics-faq.html#A6 . See also:
    R.F.W. Bader
    Atoms in Molecules
  9. Why does a filled energy level have a lower energy than a partially filled energy level?
  10. High energy in atoms mean that it is more like to interact ergo electrons have more chance to fly off an atom.

    Think about your question. An filled energy level is full. Stable. Whereas non filled energy levels have gap, and they "want" to be filled, ergo they are in higher energy.
  11. A literature tip:

    I just read the book "Absolutely small" by M.D. Fayer. It's a popular-science book trying to explain how quantum phenomena show up in everyday life. It has an extensive discussion, spanning over several chapters, of chemical bonds, without much math. It will answer exactly the question you are posing.
  12. A clarification is needed here. Two hydrogens bonded together do not form a system that looks like two isolated hydrogen atoms next to each other. When atoms bond, the electron orbitals change shape. You no longer have two different atomic orbitals, each containing one electron. You have one molecular orbital surrounding both nuclei containing two electrons. When we say "sharing electrons", you shouldn't picture a ball stuck in a tug of war between the two nuclei. Rather, both electrons spread out into the orbital surrounding both nuclei. Read up on Molecular Orbitals for more information.


    Also, two identical atoms have the same energy level for their ground state orbital wavefunction. But if they are brought close together, the Pauli exclusion principle says that the two states cannot be identical. As a result, the energy levels split. One goes slightly up in energy and one goes slighty down. Nature tends to be attracted to lower energy states, so the atoms are attracted and the bond is formed as the system assumes the lower energy state.
    Last edited: Aug 24, 2011
  13. I still don't understand why the two atoms sharing an orbital is of lower energy.
    Why is there less potential energy when there is a bond (when the electrons are in between the two atoms) as compared to the potential energy when the atoms are separate and in their lowest energy state?
    Are they on average closer to the nuclei when bonded? and thus of lower potential energy?

    Thank you
  14. DrDu

    DrDu 4,094
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    Ultimately, bonding is indeed due to electrons being closer to the nuclei,
    but the argument is quite involved.
    I urge you to have a look at the following article by Kutzelnigg or at his book on quantum chemistry if you understand german. If you don't have access,
    maybe you can find it in a library:
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