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Atomic Binding Agent?

  1. Mar 8, 2007 #1
    As I understand it, electrons orbit a nucleus in a sort of spherical "cloud", kept in tow by said nucleus' protons. However, when two or more atoms combine, what keeps the negative charge in the electron "clouds" from simply pushing them apart?
     
  2. jcsd
  3. Mar 8, 2007 #2

    Astronuc

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    The nuclei have a positive charges, each of which acts on individual electrons.

    One could ask - why don't the electrons in a given atom repel one another? Well the charge of each electron is balanced by a positively charged proton, and the protons establish an electric (Coulomb) field that keeps each electron in the appropriate orbits. Note that electrons fill particular orbitals, and once an orbital is filled, additional electrons fill in the next orbital until it is full.

    These might be useful -
    http://hyperphysics.phy-astr.gsu.edu/hbase/atomic/atstruct.html
    http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/atomstructcon.html
     
    Last edited: Mar 8, 2007
  4. Mar 8, 2007 #3
    Note that under most circumstances (as you guessed), the electron clouds do push the atoms apart. (Think of all the volume taken up by a material like copper, or of the absense of any reaction in a vial of helium despite the vigorosity with which the gaseous atoms are colliding.) Chemical bonds are exceptional situations which usually seem to depend on more complex interactions (where the electron clouds are not spherical to begin with, or a random fluctuation in one atom's electron cloud induces an accomodating fluctuation in a nearby atom's electron cloud..).
     
  5. Mar 9, 2007 #4

    Astronuc

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    It is true that an atom is mostly empty space, based on the assumption of the particle nature of the electron and the small size of the nucleus.

    I think the opposite is true. Atoms are more likely found in molecular compounds - especially H, C, N and O - as in organic chemistry.

    Metals have to be extracted from ores which are based on metal simple and complex metal oxides, e.g. carbonates, sulphates, silicates, titanates, . . . Even then, corrosion indicates that thermodynamically, metals prefer to be oxidized.

    He and the noble gases certainly are exceptional. But then look at diatomic gases, e.g. H2, N2, O2, and then halides F2, Cl2, Br2 and I2.

    Finally, in addition to the metallic bonds in metals and alloys, and covalent and ionic bonds in molecules, one has intermolecular forces - van der Waals forces and hydrogen bonds.
     
  6. Mar 9, 2007 #5
    this is rubish...what about exchange+correlation effects? exchange energies are often large, large enough that coulombically repellent electrons will occupy the same spatial orbitals! were this not the case, how could life (as we know it) even exist???

    most systems are also ground-state dominated...for example, the first excited state of the H atom is ~13.6 eV - this corresponds to many tens of thousands of degrees K. Therefore, (thermally) we could expect the vast majority of H on the SURFACE of the SUN to be in the ground state! In other words, electronic energies are HUGE!
     
  7. Mar 9, 2007 #6
    DF, I'm not a chemist; I may just be wrong.

    If I look at the oceans (mostly water and ice), then yes most of the particles are H2O molecules. But isn't electron-cloud repulsion still the reason that the oceans take up so much much volume (rather than compressing to something closer density to a neutron star)? Isn't it also the reason why one molecule usually doesn't bond with the next, why the nuclei still stay separated on the order of angstroms in the (comparatively less frequent) interactions in which a bond does form, and even why the bond angle of the water molecule is always so large (despite the natural shape of oxygen's outer orbitals)? Or should all these points be attributed directly to the repulsion of the positive nuclei?

    Q', rather than generally deriding what I wrote (but since I think I've justified the first part to Astronuc, I'll assume you're responding mainly to the last part), could you try to explain more simply (since I for one am unfamiliar with the technical jargon you've used) how to correctly understand the issue? If it's difficult to explain bonding more simply, doesn't that justify that I just described it as "more complex" than the kind of interaction raised by the OP?
     
  8. Mar 10, 2007 #7
    I don't mean to deride you personally, but what you said is simply false. There are no two ways about it.

    There is more than just electrostatic interactions - there is a full, many-bodied wavefunction that will describe what you are saying. Including it that are the, as I have pointed out, EQUALLY important effects of Pauli exchange and electron correlation - infact, it is the competition between the energy contributions of these two that give rise to what we call a chemical bond.

    A bond is not a "rare" event. You wanted simple language: it is due to the formation of nodes that arise from wavefunction interaction between the atomic orbitals that "surround" the nucleus.
     
  9. Mar 11, 2007 #8

    Claude Bile

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    When you bring two (lets assume they are identical for simplicity) atoms in close proximity of one another, the energy levels of each atom split into two. This occurs because the two atoms act as a combined system, rather than two isolated systems.

    Now, for each energy level for the isolated atom, there are going to be two for the combined, two-atom system. One of these levels will have a higher energy than the isolated atom's equivalent, the other will have a lower energy. Electrons that occupy the lower-energy level are said to be in 'bonding' orbitals because, like any system, the two-atom system will try to minimise its energy. Electrons that occupy these lower-energy levels thus cause the atoms to 'stick' together. Electrons that occupy the higher-energy level to the opposite, and are therefore said to be in 'anti-bonding' orbitals.

    When two atoms are brought in close proximity to one another, the thing that determines whether or not the atoms will 'stick' and form a molecule is simply whether the atoms' combined energy is lower that the two isolated atoms. This in turn depends on how many electrons exist in bonding orbitals and how many exist in anti-bonding orbitals. If there are more electrons in bonding orbitals than there are electrons in anti-bonding orbitals, the energy of the combined atoms will be lower and thus form a molecule.
    So to answer your question, there are conditions where the energy of two atoms close together is lower than two isolated atoms, the reduced energy due to covalent bonding overcomes the slight increase in energy due to electrostatic repulsion of the clouds.

    Claude.
     
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