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Thread: Energy and Stability

  1. #1 Energy and Stability 
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    It is often said in my chemistry book, that a reaction takes place because it makes the configuration of the product more stable than the input when combined in that way, or that it requires less energy, or occupies a lower energy level.

    Are configurations of matter more stable because they take less energy, or take less energy because they are more stable? What makes valence shells that are half full, or full, more stable than others? (In some cases, it seems the more stable element and it's corresponding valence configuration are actually more energetic than the less stable, more substantial cousin element. i.e., the lower amu element has the less stable valence config.)

    I'm coming from an amateur physicist's perspective, so for me "energy" means total relativistic energy... more electrons, protons and neutrons means more energy.

    Can someone clear this up for me?


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    Energy accumulation is information change. Relative velocity can be very high between two objects and as long as nothing else about them is changing, the energy levels of each are static. It's only when you decelerate or accelerate one of the objects that energy levels change, and that is because one of the objects is changing. (Of course this is how you can tell who is doing the changing between two objects, or in what portions each are changing in a simple system just modeling acceleration.)

    Chemical energy accumulation (thermodynamics) and loss is what I'm looking for, something akin to the physical theory of why an input of energy into one object will be expressed as a change in momentum.

    Chemistry textbooks that I am going through (these are university, general chemistry) give just an overview of the rules and an explanation on how those rules work.

    There hasn't been anything presented yet that explains (any attempt at all) how the qualities of electron orbits are achieved for specific atoms in molecules. It is mentioned along side that it happens to be more stable and that it happens to be lower energy, but it doesn't say why the process should prefer that or how it would be guided into it by a subset of rules that govern how they relate to physics.

    Is this what physical chemists study? Or where would I look?


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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Adam144 View Post
    There hasn't been anything presented yet that explains (any attempt at all) how the qualities of electron orbits are achieved for specific atoms in molecules. It is mentioned along side that it happens to be more stable and that it happens to be lower energy, but it doesn't say why the process should prefer that or how it would be guided into it by a subset of rules that govern how they relate to physics.
    I guess that is because it is pretty damn complicated. This is where physical chemistry meets quantum mechanics. It is, in principle, possible to calculate the relative strengths and energies of different bonds from first principles (i.e. what is known of the properties of electrons, etc.). In practice, this can only be done by using various approximations and numerical methods. This is done in computational chemistry, for example, to work out how novel compounds will react (and how proteins will fold, etc). I don't remember the names of any of the methods used, other than Hartree-Fock ...
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    Even if I won't understand it, it's good to know whether others do or not. What's the reason for atoms with exceptional electron configurations (to make half-full levels, etc.)... is it understood? Am I correct to interpret "It is more stable" as a statement about observed effects rather than a cause for that configuration?

    One way I heard it explained by another amateur was that energy is hard to earn, so the exceptional valance configurations are just as far along as that particular bundle of protons got at "earning" electricity.
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Adam144 View Post
    Am I correct to interpret "It is more stable" as a statement about observed effects rather than a cause for that configuration?
    At one level it is a cause. You can get a long way in chemistry by just taking minimising energy / becoming more stable as the mechanism (and a set of apparently ad hoc rules about electronegativity, etc).

    On the other hand, you can see this as a result of the underlying mechanisms which are due to quantum mechanics (Pauli exlcusion principle, etc).

    You can always ask more "why" questions. Every cause is a result of something else. Sooner or later you get to "that's just the way it is" (aka, we don't know). There is a good video online of Feynman talking about this (with regard to magnetism, I think).

    It is years since I studied this (as physical chemistry rather than quantum mechanics) so I won't attempt to answer in more detail!
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    Quote Originally Posted by Adam144 View Post
    Even if I won't understand it, it's good to know whether others do or not. What's the reason for atoms with exceptional electron configurations (to make half-full levels, etc.)... is it understood? Am I correct to interpret "It is more stable" as a statement about observed effects rather than a cause for that configuration?

    One way I heard it explained by another amateur was that energy is hard to earn, so the exceptional valance configurations are just as far along as that particular bundle of protons got at "earning" electricity.
    In tracing the "why" questions that Strange refers to, you may find it worthwhile to look at what Pauling did: Linus Pauling and The Nature of the Chemical Bond: A Documentary History - Special Collections - Oregon State University

    He won the first of his Nobel prizes for this work.

    I first read his initial paper on the subject when I was an undergraduate. I was unable to follow. As a grad student, I returned to it and was shocked at the many ad hoc assumptions he made; it seemed that he was forcing quantum theory to produce results he already knew had to be correct. I came to understand that this was necessary to make progress, particularly in a slide-rule age.
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