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Thread: Atomic model

  1. #1 Atomic model 
    Forum Masters Degree thyristor's Avatar
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    Hi!

    As I have understood it, the problem with the pre-Bohr atomic model was that it postulated circular motion for the electrons around the nucleus, which would lead to a constant emission of photons and so also energy due to the acceleration of the electrons. This would result in them losing kinetic energy and thus constantly approach the nucleus.

    Bohr's model postulates that the electrons move at fixed energy levels, and that they cannot continuously decrease their velocity and approach the nucleus, as would be the case in the pre-Bohr model. However, I cannot see how this solves the paradox. Electrons are still postulated to move around the nucleus - I believe this is one of the factors contributing to the magnetic moment of an atom - and would thus constantly accelerate and thus emit photons and lose energy.

    Any attempt at clearing the fog that clouds my mind is welcome.


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  3. #2 Point_particle 
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    http://en.wikipedia.org/wiki/Point_p...ntum_mechanics

    In quantum mechanics, there is a distinction between an elementary particle (also called "point particle") and a composite particle. An elementary particle, such as an electron, quark, or photon, is a particle with no internal structure, whereas a composite particle, such as a proton or neutron, has an internal structure (see figure). However, neither elementary nor composite particles are spatially localized, because of the Heisenberg uncertainty principle. The particle wavepacket always occupies a nonzero volume. For example, see atomic orbital: The electron is an elementary particle, but its quantum states form three-dimensional patterns.


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    Forum Masters Degree thyristor's Avatar
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    Although I appreciate your contribution, I fail to see how the provided link answers my question. Whether this is due to an inability on my part to comprehend the material, or if the material does in fact not answer my question I don't know. In any case, I would appreciate if you could elaborate a bit.
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    As you described correctly, a classical system with accelerated charges would constantly lose energy by radiation. This is the synchrotron radiation that is e.g. produced in particle ring colliders. While losing energy, the radius of the electron orbit must decrease, classically speaking.

    The postulate is that the electrons can only attain certain fixed energy levels. This ad hoc condition prevents the continuous emission of radiation and reduction of the classical orbital radius. If an electron "wants" to lose energy, it can only do so in certain packets or "quanta", hence the expression "quantum mechanics". So, electrons can only jump between energy levels within an atom. There is a minimum energy that can be attained in an atom. If this is higher than the next allowed jump to the "centre" of an atom, it cannot do it and stays at the lowest allowed energy level.

    There is another postulate that prevents electrons to gather up in the lowest energy level - the Pauli principle. It prohibits to have two particles at the same energetic state. So, even the two electrons at the lowest orbital differ in energy - in this case it's the spin. There are other "quantum numbers" that had to be defined leading to a greater diversity in energetic states in higher oribtals (see Bohr-Sommerfeld theory), still using a semi-classical model that includes solid particles and planet-like orbits (modern quantum mechanics has abandoned this model).

    So, as you see, the basics of quantum mechanics does not actually explain, why the electron does not attain continuous energy levels, but sets up the rules how the electron behaves.
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    Forum Masters Degree thyristor's Avatar
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    But electrons are still postulated to move around around the nucleus, right? How is this possible without the electrons constantly losing and energy? I understand what you're saying about the postulate that they cannot lose energy continuously, but to me it still seems to violate the law that a charged article udnergoing acceleration will emit a quantum of energy, a photon.
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  7. #6  
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    Quote Originally Posted by thyristor
    But electrons are still postulated to move around around the nucleus, right? How is this possible without the electrons constantly losing and energy? I understand what you're saying about the postulate that they cannot lose energy continuously, but to me it still seems to violate the law that a charged article udnergoing acceleration will emit a quantum of energy, a photon.
    "Moving around the nucleus" is a classical simplification. A better description is to consider electrons as "waves" or being at discrete levels. The Pauli principle restricts the levels that electrons (other than in H) can occupy.
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  8. #7  
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    Quote Originally Posted by thyristor
    But electrons are still postulated to move around around the nucleus, right? How is this possible without the electrons constantly losing and energy? I understand what you're saying about the postulate that they cannot lose energy continuously, but to me it still seems to violate the law that a charged article udnergoing acceleration will emit a quantum of energy, a photon.
    As already mentioned, the classical picture of a planetary system equivalent is incorrect. You should abandon that. Electrons are not to be thought of as solid particles that have a certain shape and velocity. We now describe an electron as something that has a certain probability to be anywhere around the nucleus, with the probability being higher at certain locations. One could even imagine the electron to be smeared out around the nucleus instead of being somewhere at a certain time. So, the visualisation of an electron revolving around the nucleus is wrong. It provides nearly correct results in the case of a hydrogen atom, but totally wrong results for others. This is the reason, why the configuration of electron orbitals does not contradict the classical analogy of revolving, hence radiating charged particles. It is just wrong. Quantum mechanics is weird! The picture shows you a list of possible electron orbital configurations (i.e. iso-probability zones).

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    how can an atom have a current without emitting light?
    superconductors also have current without any resistance.


    And, of course, atoms DO emit light when electrons transition to a lower energy level.

    this can be compared to the alternating current of a josephson junction where




    where U(t) and I(t) are the voltage and current across the Josephson junction, φ(t) is the "phase difference" across the junction (i.e., the difference in phase factor, or equivalently, argument, between the Ginzburg-Landau complex order parameter of the two superconductors composing the junction), and Ic is a constant, the critical current of the junction

    from:
    http://en.wikipedia.org/wiki/Josephson_effect
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  10. #9  
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    Bohr's model postulates that the electrons move at fixed energy levels, and that they cannot continuously decrease their velocity and approach the nucleus, as would be the case in the pre-Bohr model. However, I cannot see how this solves the paradox. Electrons are still postulated to move around the nucleus - I believe this is one of the factors contributing to the magnetic moment of an atom - and would thus constantly accelerate and thus emit photons and lose energy.
    first of to understand this concept you need to forget about classical model and need to enter into quantam concept.i think you to need to enter to the de Broglie concept of wave particle duality.thus electron being the wave froms an stationary wave around the nucleus in the fixed orbit also you might know that in case of stationary wave, energy is confined in fixed place which facillate the stationary energy level.further this concept doesnt help for line spectra and hence sommerfeld introduce new concept elleptical orbit.i only gave the little explanation so i think you need to search about stationary wave around nucleus to get more detail.there are some mathmatics which will help u as well. http://www.kentchemistry.com/links/A...sElectrons.htm
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  11. #10  
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    Quote Originally Posted by thyristor
    But electrons are still postulated to move around around the nucleus, right? How is this possible without the electrons constantly losing and energy? I understand what you're saying about the postulate that they cannot lose energy continuously, but to me it still seems to violate the law that a charged article udnergoing acceleration will emit a quantum of energy, a photon.
    The radiation of energy by an accelerated particle is a prediction of classical electrodynamics.

    The behavior of electrons in the atom is the domain of quantum electrodynamics.

    The violation of classical electrodynamics by the electron in early atomic models was one of the clues that led to quantum mechanics.
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  12. #11  
    Forum Masters Degree thyristor's Avatar
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    Thank you everybody for your helpful replies! I now realize that what my question probably really was related to, was the first statement in Dr Rockets post, namely if the radiation of energy by an accelerated particle was a prediction of classical electrodynamics, and I had my answer.

    To spin off on this, how does one explain brehmsstrahlung using quantum mechanics? Obviously we cannot say it is due the the acceleration of charged particles as this is a prediction of classical electrodynamics.
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  13. #12  
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    Quote Originally Posted by thyristor
    To spin off on this, how does one explain brehmsstrahlung using quantum mechanics? Obviously we cannot say it is due the the acceleration of charged particles as this is a prediction of classical electrodynamics.
    Google is your friend.http://en.wikipedia.org/wiki/Bremsstrahlung

    Also remember that classical electrodynamics is still valid, so long as you deal with situations that are describable classically. Electron orbitals in the atom are not. Electron trajectories outside of the atom may often be accurately described in classical terms.

    There are a hell of a lot of things that are readily treated classically but, while in principle described by quantum electrodynamics, are in practice out of reach because nobody can solve the equations.
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  14. #13 Re: Atomic model 
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    Quote Originally Posted by thyristor
    Bohr's model postulates that the electrons move at fixed energy levels, ...
    No experimental proof of this "moving" of something that would be much smaller than the entire atom.

    Quote Originally Posted by granpa
    http://en.wikipedia.org/wiki/Point_particle#In_quantum_mechanics
    an elementary particle (also called "point particle")
    No experimental proof of this alleged "punctual character".
    And no experimental proof that ouf familiar macroscopic space, could be autosimilar at any scale, even at quantic scale.

    Quote Originally Posted by Dishmaster
    ... a classical system...
    Er... "Classical" in the parlance of this tribe means "obsolete, before us".
    So, without ever defining what distinguishes macrophysics from microphysics, they define themselves as the modern people for ever after.

    Quote Originally Posted by thyristor
    But electrons are still postulated to move around around the nucleus, right? ...
    Simultaneously yes and no. So are the practiced postulates.

    Quote Originally Posted by Dishmaster
    ... We now describe an electron as something that has a certain probability to be anywhere around the nucleus, with the probability being higher at certain locations.
    The concept of "probability" is engineered to accomodate a strictly undulatory formalism, with a still corpuscularist semantics. Engineered to hide the confusion between two scales of analysis, distant of at least five orders of magnitude.

    Quote Originally Posted by saurab dulal
    I think you to need to enter to the de Broglie concept of wave particle duality.
    Yes, Louis de Broglie was na´ve and conservative enough to maintain the macroscopic concept of "corpuscle", the macroscopic concept of "spatial coordinates", the macroscopic concepts of macroscopic space and time, though no one of them have ever proven valid in the microphysics field...

    However, he was the one who invented the concept of intrinsic frequency .

    Moreover, remaining within the relativistic framework, Broglie stated the theorem of Harmony of the phases, and proved that the product of the phase velocity and the group speed of an electron, is the square of light speed :


    http://deonto-ethics.org/mediawiki/i...nce_broglienne
    http://deonto-ethics.org/mediawiki/i...nde_de_Broglie


    This implies, but Broglie himself did not see it, that in the electron frame, its phase velocity is infinite, so the electron is everywhere in phase with itself in its spatial extension. So its phase spatial extension can never become "very small", and can never become corpuscular, nor "dual", nor "punctual".

    This implies, too, that no electron nor nucleon could be isolated from the noise of all the other broglian noises of all the other quantons. John G. Cramer missed the fact the this unvaidable broglian noise is the main physical background that provides the handshakes between absorbers and emitters. As any other american physicists, Cramer does not read french, and the thesis of Louis de Broglie was never translated into english. That's so... What is not published in english is meant not-being.

    http://deonto-ethics.org/mediawiki/i...ansactionnelle
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