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Math Help - Bohr Model

  1. #1
    Senior Member DivideBy0's Avatar
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    Bohr Model

    Can someone please check this to see if it is correct? It's for an assignment

    In 1913, Danish physicist Niels Bohr solved this problem, by proposing yet another model of the atom. In this model, electrons would be restricted to orbiting in atomic shells. As the electron orbited around the nucleus, it would eventually lose energy, due to attraction, and would move to a lower energy state i.e. an atomic shell closer to the nucleus. Eventually, when the electron reached the lowest possible energy state, it would give off a photon and return to a higher energy state, further away from the nucleus.

    I don't quite know why or how the electron returns to its higher energy state, hope someone can explain to me. No fancy formulae please. This is just the start of my chemistry unit so all I know so far is just from self-research. Thanks
    Last edited by DivideBy0; August 26th 2007 at 03:31 AM.
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    Grand Panjandrum
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    Quote Originally Posted by DivideBy0 View Post
    Can someone please check this to see if it is correct? It's for an assignment

    In 1913, Danish physicist Niels Bohr solved this problem, by proposing yet another model of the atom. In this model, electrons would be restricted to orbiting in atomic shells. As the electron orbited around the nucleus, it would eventually lose energy, due to attraction, and would move to a lower energy state i.e. an atomic shell closer to the nucleus. Eventually, when the electron reached the lowest possible energy state, it would give off a photon and return to a higher energy state, further away from the nucleus.

    I don't quite know why or how the electron returns to its higher energy state, hope someone can explain to me. No fancy formulae please. This is just the start of my chemistry unit so all I know so far is just from self-research. Thanks
    Without absorbing energy from somewhere it cannot return to a higher
    energy level (except for the breif time allowed by the uncertainty principle).

    RonL
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    Quote Originally Posted by CaptainBlak View Post
    (except for the breif time allowed by the uncertainty principle).
    We never did the Uncertainly Principle in high school. I assume the poster never done it as well.
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    Senior Member DivideBy0's Avatar
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    So, if an atom doesn't receive any energy, wouldn't it eventually collapse in on itself?
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    Bar0n janvdl's Avatar
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    This is probably totally wrong, but if it gives something off, its mass would decrease right? And the force that attracts it would thus be less, and it would move further away from the nucleus... right?
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    Senior Member DivideBy0's Avatar
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    I think that might be true, but the force I'm more worried about is the electromagnetic force, not the gravitational force, if that's what you meant
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    Quote Originally Posted by DivideBy0 View Post
    So, if an atom doesn't receive any energy, wouldn't it eventually collapse in on itself?
    No. There are several states an atom might have. For example, take Hydrogen, I believe it has 11 possible states. (See below). I only drew 5 states. Now the blue electron is on state #5. It is not gaining energy so it will drop down a state and get closer to the nucleus. But when it does that it loses energy. That is the photon it gained. If nothing gives it energy it will continue doing this until it reaches the most bottom state (called ground state). Again it will give of energy (photons) when it reaches that state. At groud state it cannot keep going down and it shall stay like that. Until energy is absorbed again by the electron. That causes it to move to a higher state. (This is all happens in 1 billionth of a second).
    Attached Thumbnails Attached Thumbnails Bohr Model-picture33.gif  
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    Grand Panjandrum
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    Quote Originally Posted by DivideBy0 View Post
    So, if an atom doesn't receive any energy, wouldn't it eventually collapse in on itself?
    No because it cannot decay into a lower energy state than its ground state.

    RonL
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    Forum Admin topsquark's Avatar
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    Quote Originally Posted by DivideBy0 View Post
    t

    In 1913, Danish physicist Niels Bohr solved this problem, by proposing yet another model of the atom. In this model, electrons would be restricted to orbiting in atomic shells. As the electron orbited around the nucleus, it would eventually lose energy, due to attraction, and would move to a lower energy state i.e. an atomic shell closer to the nucleus. Eventually, when the electron reached the lowest possible energy state, it would give off a photon and return to a higher energy state, further away from the nucleus.
    What is going on in that last statement?? Is this something you wrote or is a quote from an article?

    Where ever it came from, it is incorrect.

    Bohr proposed that the electrons circled the nucleus in semi-stable states. That is to say if an atom is in an excited state the electron could give off a photon, but would have to give off energies of only certain amounts; the electron would be required to "jump" down the energy scale in "quanta" of energy until it reached a lowest, or "ground" state. There is nothing to say that an electron in the ground state will get re-elevated to an excited state again.

    This was a radical idea since Maxwell's equations proved that a charge moving in a circle will continuously give off electromagnetic radiation and spiral into the center. Bohr's electrons didn't do that. They emitted energy if in an excited state and "spiraled" in to a lowest state, but they stayed in that lowest state and didn't spiral in any further. It was a completely preposterous model given the state of Science at the time, but couldn't be discarded since it worked to an extremely good approximation.

    (By the way, Bohr's model of the atom was in his PhD dissertation. His mentors had assigned him the "impossible" problem of explaining line spectra to embarrass him and show him how much more he had to learn about science. He was, in fact, completely wrong. But since his results were so accurate nobody could explain that he was wrong until Schrodinger (and others) solved the wave equation for Hydrogen in the late 1920s. And the model still remains very useful today to introduce the topic of atomic spectra and atomic orbitals. Guess his mentors showed him, didn't they? )

    -Dan
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    Senior Member DivideBy0's Avatar
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    Thanks... so the Bohr Model still hadn't solved the spiralling in of electrons, it was just 'better' than the Rutherford Model in that it was more accurate. I thought it seemed a bit odd that the electron doesn't fall lower than ground state.
    How does this link in with Pauli's exclusion principle though (or isn't it supposed to)? i.e. An electron falls to the ground state, and the atom doesn't receive any energy. After a while, the other electrons in their orbits also lose energy and fall to ground state. Wouldn't that result in multiple electrons in the same atomic shell?
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    Forum Admin topsquark's Avatar
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    Quote Originally Posted by DivideBy0 View Post
    Thanks... so the Bohr Model still hadn't solved the spiralling in of electrons, it was just 'better' than the Rutherford Model in that it was more accurate. I thought it seemed a bit odd that the electron doesn't fall lower than ground state.
    How does this link in with Pauli's exclusion principle though (or isn't it supposed to)? i.e. An electron falls to the ground state, and the atom doesn't receive any energy. After a while, the other electrons in their orbits also lose energy and fall to ground state. Wouldn't that result in multiple electrons in the same atomic shell?
    More or less the Bohr atom was the origin of the Pauli exculsion principle. The Bohr atom requires that the H- ion, for example, have two electrons in the same shell in the ground state. The H-- ion had two electrons in the same lowest shell, but one electron in higher level, etc. It was determined that the only way to explain the way the shells filled was to postulate the existence of a new quantum number, spin, and say that two electrons in an atom could not share the same quantum numbers. From this it was derived that the spin of an electron had the "nonsensical" value of 1/2, meaning that two electrons could share the same n, l, and ml quantum numbers (and have opposing spins.)

    It was later taken as a postulate of quantum mechanics that no two fermions (half-integer spins) could share the same quantum numbers (this is the Pauli exclusion principle), but any number of bosons (integer spins) could have the same quantum numbers. This was eventually proven using a statistical version of quantum mechanics. No particles have ever been found that are neither fermions nor bosons.

    -Dan
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    Quote Originally Posted by DivideBy0 View Post
    Thanks... so the Bohr Model still hadn't solved the spiralling in of electrons, it was just 'better' than the Rutherford Model in that it was more accurate. I thought it seemed a bit odd that the electron doesn't fall lower than ground state.
    It does in that action is quantised (which was Bohr's hypotheseis), and that leaves the electron unable to drop into a lower energy state.

    RonL
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    Senior Member DivideBy0's Avatar
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    Thanks for the help, but I guess I'll just need to take more chemistry and physics in order to fully appreciate how the model works, some things are over my head.
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    Forum Admin topsquark's Avatar
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    Quote Originally Posted by DivideBy0 View Post
    Thanks for the help, but I guess I'll just need to take more chemistry and physics in order to fully appreciate how the model works, some things are over my head.
    Just give it some time and keep asking questions. You'll get there.

    -Dan
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    Quote Originally Posted by DivideBy0 View Post
    Can someone please check this to see if it is correct? It's for an assignment

    In 1913, Danish physicist Niels Bohr solved this problem, by proposing yet another model of the atom. In this model, electrons would be restricted to orbiting in atomic shells. As the electron orbited around the nucleus, it would eventually lose energy, due to attraction, and would move to a lower energy state i.e. an atomic shell closer to the nucleus. Eventually, when the electron reached the lowest possible energy state, it would give off a photon and return to a higher energy state, further away from the nucleus.

    I don't quite know why or how the electron returns to its higher energy state, hope someone can explain to me. No fancy formulae please. This is just the start of my chemistry unit so all I know so far is just from self-research. Thanks

    It is the other way around. An electron is excited and aquires more energy it goes to a higher shell. At a later time it can fall back to it's original lower energy shell and gives off a photon.The emmision of a photon balances the energy equation and the energy of the photon is the same as the difference in potential energy between the two shells.

    The electron can only exist in the shells and not between them so it "quantum leeps" from one shell to another. This means electrons in an atom can only absorb or give of energy (photons) in whole "quantum chunks". Look up Planck's constant and Einstiens photoelectric experiment to learn more about this stuff. Then again you might not want to go there.
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