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Electostatic vs Electromagnetic

  1. May 7, 2016 #1
    Greetings. I am a new member posting for the first time. I teach chemistry and I have spent considerable time attempting to get a definitive answer to this question but have found only contradicting information. My query is regarding the nature of the electric force within and between atoms and molecules. In order to explain bonding I explain the difference between an electrostatic and an electromagnetic force to be if the particles are moving relative to one another. As such, I explain the force between protons and electrons to be electromagnetic since electons are moving. The problem I have arises when explaining intramolecular bonding. First, why are ionic and metallic bonds generally classified as intramolecular rather than interatomic? Second, if the force in a metallic bond is electromagnetic since it is between cations and moving delocalized electons, and the force between ions is electostatic since the ions are not moving, what type of force exists in a covalent bond? Since the attraction holding the atoms together is between the atoms and the shared electons placed in the molecular orbital, isn't it actually electromagnetic? Thanks in advance to anyone who takes the time to read this lengthy post and indulge my ignorance by providing an answer!
     
    Last edited: May 7, 2016
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  3. May 7, 2016 #2

    jfizzix

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    Welcome to PhysicsForums :)

    Are inter-atomic (between atoms) and intra-molecular (within a molecule) kind of the same thing? Is one a subset of the other?

    I only took freshman college chemistry, so there may be a bit of "physics hubris" at play in my answer. I think that the electrostatic force primarily determines how chemical bonds work because magnetic effects are several orders of magnitude weaker. Whether moving or not, I believe it's the electric field (rather than the magnetic field) generated by the charges that eventually describes the chamistry.

    If you're going to try and have a classical-physics picture of chemical bonding, it's going to be an uphill battle. The closest I can think of would be a moon orbiting a planet. In the rotating frame of reference, you see that the circular orbit is at a minimum of energy, and any slight disturbance causes the orbit to oscillate about that minimum (giving you an elliptical oribit). In that sense, the moon and planet are bound to one another.

    It might actually be a bit simpler to include a smidgen of quantum physics when describing chemical bonding, and some of the students will no doubt perk right up when you start talking about quantum physics. Keeping in mind that this is my own recollection and reasoning, you'll want to corroborate this elsewhere. Also, I apologize in advance if this is all painfully familiar to you and I just completely lost the point of your question.


    (The overly simple model of a chemical bond)
    We can look at the relatively simple case of a diatomic bond (as in molecular hydrogen)
    There, we have two protons with positive change, and two electrons with negative charge.

    When the atoms are far away, each electron orbits its proton, and you have two isolated hydrogen atoms. The electrons are well treated as hazy clouds of probability surrounding each proton.

    As the distance gets smaller, the electron clouds will start to overlap, so that parts of the electron cloud of one atom will be within the electron cloud of the other atom. The electrons experience a net attractive force because there is less screening of the field of the protons as they get closer. The protons also experience a net attractive force, because of the increased negative charge between them,

    When they get closer still, the protons experience a net repulsive force because again, their fields aren't being screened by their respective electron clouds, and the electrons don't concentrate exclusively between the two protons. Because of this, there is some critical distance where there is no net attractive or repulsive force between the two atoms, and this distance defines the bond length.

    So in the overly simplified model of a chemical bond, magnetic effects don't necessarily come into play.

    However, there are other effects that contribute to the strength of these bonds. One example is that these probability clouds aren't necessarily static in time, the result of this is that sometimes by random chance, a single neutral hydrogen atom will for an instant have a nonzero electric dipole moment, simply because the cloud had a slightly higher concentration on one side of the atom, than on the other. These fluctuations are random, and rapidly reverse back and forth, but on average, there is "some" dipole moment at most instances in time; this means that two neutral atoms (or molecules) will experience a net attractive force just due to these random fluctuations amounting to an attractive force between two electric dipoles. These forces are called London dispersion forces and are responsible for why Bromine is a liquid at room temperature (instead of a gas).

    Also, as far as the distinction between ionic, covalent, and metallic bonding goes, real bonds need not be so distinctly classified. See for example:
    https://en.wikipedia.org/wiki/Van_Arkel–Ketelaar_triangle
     
    Last edited: May 7, 2016
  4. May 7, 2016 #3
    Thank You for providing such a detailed answer! I always attempt to anticipate questions regarding what I am presenting and, to that end, I can't seem to find any definitive answer regarding the difference between electrostatic and electromagnetic forces with regard to bonding. The terms seem to be used synonymously in some instances yet I know they are not interchangeable. Maybe I am simply over-thinking it???
     
  5. May 7, 2016 #4

    jfizzix

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    All forces due to charges, moving or otherwise can be called electromagnetic.

    The electromagnetic field can be decomposed into an electric field and a magnetic field, both of which can be changing in time and influencing each other as described by Maxwell's equations.
    The electric field is a field generated by charges according to their positions.
    The magnetic field is a field generated by charges according to their velocities.
    A distribution of charges held fixed and motionless generates an electric field, but no magnetic field. The force felt by a charge due to a static electric field is an electrostatic force, which is one kind of electromagnetic force.
     
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