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Thread: Why do atoms want to gain full outer shells?

  1. #1 Why do atoms want to gain full outer shells? 
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    Why to atoms move electrons to form full outer shells? What are the benefits of having a full outer shell?


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    Having a full outer shell makes an atom more stable. Other than that, the only real answer is..."they just do."


    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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  4. #3 Re: Why do atoms want to gain full outer shells? 
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    Atoms with a full (or empty) outer shell (generally) do not react as readily, making them more stable.
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    Simply put, atoms want to be happy. Full outer shells make atoms happy.
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    As with a lot of chemistry, this question has a simple answer, and a more complicated quantum mechanical answer. The simple answer is something along the lines of, the atom wants to gain or lose electrons to approximate the electronic configuration of the nearest noble gas, which are inherently stable.

    Of course that doesn't really explain why they want to have this configuration. The answer to this lies in quantum mechanics, and unfortunately I'm a little hazy on that myself, so perhaps someone else could explain better. The basics of it rely on Hund's first rule. What this basically boils down to is that since two electrons cannot share the same set of quantum numbers (though I think there are exceptions) they fill the orbitals one at a time with one spin state, then go back and fill the orbitals with the other spin state. The lowest energy configuration is when all the positions are filled.
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    farmboy, I haven't heard it put that way before but it seems to be a reasonable explaination.
    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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    Correct me if I'm wrong. I think the explanation is that Z-effective, which decides electron affinity depends on the core electrons, not the valence electrons. eg. Fluorine has 9 protons and 2 CORE electrons. Z-effective = 9-2 = 7 therefore fluorine has a high electron affinity and sucks up electrons. But if you add an electron the valence electrons become core electrons. Z-effective = 9-10 = -1. Fluorine is no longer has a large electron affinity and therefore stops absorbing electrons.

    As for metals like sodium, they are not more stable in ionic form, which is why they only react when water, halogens and other substances with a high electron affinity (which suck up all the metal's electrons) are nearby. Na Z-effective = 11-10 = 1. Na+ Z-effective = 11-10 = 1. Na2+ Z-effective = 11-2 = 9 > 7 = F Z-effective . Therefore Na2+ theoretically has a greater electron affinity than fluorine, so if Na2+ is ever created it will quickly regain any electrons lost, this is the same for all alkali and earth alkali metals.

    Besides the personification of atoms, to say that atoms "want to be like nobel gases" may be misleading. Maybe it's more accurate to say that atoms want to have as many electrons as possible, but aren't strong enough to hold on to any more than nobel gases can. For metals it's different, they want as many electrons as possible, but they keep getting ripped off by other atoms, but they are strong enough to hold on to the number of electrons in the nearest nobel gas.
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    Well, electronegativity is a measure of the atom's attraction for electrons in a bond...so you can't really say that it's "no longer very electronegative". And atoms don't suck up electrons because they're electronegative, the measure of 'sucking power' if you'll excuse the term is electron affinity. Good to put that much thought into it though, I wouldn't have.
    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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    Quote Originally Posted by Chemboy
    Well, electronegativity is a measure of the atom's attraction for electrons in a bond...so you can't really say that it's "no longer very electronegative". And atoms don't suck up electrons because they're electronegative, the measure of 'sucking power' if you'll excuse the term is electron affinity. Good to put that much thought into it though, I wouldn't have.
    Oops. I've changed electronegativity to electron affinity, since electronegativity doesn't fit with anything I said. Sorry. Is that more accurate or do you think there is another problem with it?
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  11. #10  
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    Quote Originally Posted by Golkarian
    Quote Originally Posted by Chemboy
    Well, electronegativity is a measure of the atom's attraction for electrons in a bond...so you can't really say that it's "no longer very electronegative". And atoms don't suck up electrons because they're electronegative, the measure of 'sucking power' if you'll excuse the term is electron affinity. Good to put that much thought into it though, I wouldn't have.
    Oops. I've changed electronegativity to electron affinity, since electronegativity doesn't fit with anything I said. Sorry. Is that more accurate or do you think there is another problem with it?
    Have you studied quantum mechanics yet. It is a subject I wish I understood better, but can't unfortunately. Quantum mechanics explains this situation properly, I've seen the explanation though I can't repeat it.
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    Only in first year chemistry, they explained it this way, but the prof didn't go into the quantum mechanics of it. The quantum mechanics was just basic stuff like electron config, hybrid orbitals, etc. Do you remember if quantum mechanics explained it more completely or more accurately than this? Wish I could see the full explanation though since alot of theories in chemistry are good but inferior to quantum mechanics. Just so I know if my explanation is an inferior but reasonable explanation or complete crap.
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  13. #12  
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    Golkarian and farmboy are correct. It all has to do with how much positive charge from the nucleus the electrons in the outer shell experience (which is usually called Z-effective or Zeff), which depends on the principle quantum numbers of the electrons involved.

    The octet rule occurs mainly because an electron around a nucleus will not perfectly "shield" another electron from the nucleus's positive charge, especially if the shielding electron and the incoming electron have the same principle quantum number. 2p electrons can't shield other 2p electrons very well (and 3p can't shield other 3p very well, etc.). It's kind of complicated, but I will explain it as best I can.

    Consider a helium atom and a hyrogen atom (a proton). If I only give my He atom one electron and give my proton no electrons, both my He and my proton will have a charge of +1. Since they have the same charge, you might think that an electron would be equally attracted to either one - but that's not the case. An electron is much more strongly attracted to an He+ atom than to a H+ atom. This occurs because even though the He+ and the H+ have the same charge, the He has two protons and the electron that's already present won't perfectly shield the incoming electron from one whole unit of positive charge. The result is that an electron coming into an He+ atom will experience a positive charge that's something like +1.3 insted of just +1. How well an electron shields an outer electron from the nucleus depends on the principle qunatum number and angular momentum quantum number of the electrons involved. If an electron has the same n and l value as the electron that it's trying to shield, it won't be able to shield very well.

    Consider a neutral carbon atom: it has 6 protons and 6 electrons (2 1s electrons, 2 2s electrons, and 2 2p electrons). If I add a new electron to make a C- anion, the new electron that I'm adding will think that the atom has a charge of around +0.6 because each of the 2p electrons that are already there can only shield another 2p electron from about .7 units of charge. But if I want to add an electron to a F atom to make F-, now my additional electron will see a charge of something like +1.5, since there are already 5 2p electrons present that each allow +0.3 charge to "bleed through" their coverage of the nucleus. If I want to add another electron to my F-, now I will have to add a 3s electron, and the 2p electrons that are already there will shield the 3s electron much better than they can shield other 2p electrons. So the first extra electron that you add to F will see a charge of around +1.5, while the second will see a charge close to -1.

    That is the main reason why atoms are more stable if they can get to 8 electrons to make an octet; so long as you are filling up a partly-filled p orbital, positive charge from the nucleus will be able to get through to attract the extra electrons. Once you have filled the p orbital completely, you now have to add to the next level s orbital, to which very little extra charge from the nucleus can get through. There are also a few issues with electrons being lower in energy if there are a lot of other electrons around with the samle n, l, and Ms values that contribute to the octet rule, but it mainly has to do with charge and charge shielding.
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  14. #13  
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    they want to be as cool as the noble gases, who are already filled. It's like I've got 4 ho's, you've got 8 and you're jealous..you want to get more ho's of your own so you can run a pimp game equal to the best elements out there..the stable and strong noble gases.
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    this is just my guess, but even though QM say that there are different rules down at sub atomic levels, i believe that the electrons behave that way because of centrifugal forces, and therefore do not want to fill the outer shell but have to because they are moving so fast.
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  16. #15  
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    Main problem there, although there are a few, is that electrons do want full outer shells.
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  17. #16  
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    Short answer - because they do

    Long answer - atoms don't actively try for a full outter shell, they just react wth any nearby substance, eventually it will react with something causing both elements to have full outter shells, this causes the atoms to stabalize and stop reacting, allowing us to find copper carbonate for example. the reason we don't usually find unstable compounds is for the simple reason that they are still unstable and so have continued to react and eventually formed a stable compound long before we got there



    there are other conditioned reasons, for example if you have done enough chemistry to know about the 1s2,2s2,2p6,3s2 bit etc etc (the name eludes me at the moment) you will also know electrons move around in oppositely rotating pairs (equal rotation causes the electons to destroy each other) if an electron is unpaired it's electrostatic charge will attract an equally lonely compatable electron (it's like a dating site for atoms ¬_¬ ) so any element with a odd number of electrons (especially group 7 and 1) in the outter shell will technically actively try to gain or lose electrons
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    Thank you Scifor refugee fro a comprehensive and wonderfully explanatory response.

    I wish there some way we could easily archive it.
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    I think it all just depends on the orbitals that are filled yet...
    Somehow it could really be expressed by shielding. the electrons have to fit around the core in a way the don't destabilize each other. The best way to do so is to get regular positions around the core, for example, geometrically spoken 2 in the X-Axis, 2 in Y and 2 in Z... for each energy level (period) 2 electrons have a whole orb around the core in the period-specific radius.
    that would be the explanation for the first two periods, the higher orbitals will have to fit in so that no electrons destabilize.
    Tell me if you don't agree, I got to this theory out of core-physics
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  20. #19  
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    I think quantum physics explains how an atom gains stability and the order in which the electrons fill up but not the reason why it tends to become stable
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  21. #20  
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    Thanks for resurrecting a 5 year old thread to answer a question that was already answered.

    Welcome to the forum.
    dan hunter likes this.
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
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