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Octet production, why?

  1. Jul 28, 2011 #1
    Just a Quick question, do atoms 'want' to form octets due to electronegativity varying through periods, or is it due to there wanting to be an equilibrium between electrons... i.e. paired orbitals, and all quantum numbers filled. I think I am brain dead at the moment so... I'm probably just not thinking :/

    Thanks guys!
     
  2. jcsd
  3. Jul 29, 2011 #2
    The Octect rule says that the electrons will have a tendency to have 8 electrons in the valance shell. I've never really heard it discussed in the context that you present the question.
    By looking at the valence electrons of the atoms in the molecule, you simply add them up. You will find a lot of compounds obey the Octect and a lot of them don't. Boron and Beryllium are examples. Try Boron Fluoride and Beryllium Fluoride. Add up the electrons in the compound and then form the bonds around the central atom, then distribute the remaining electrons to the peripheral atoms, adding any extra electrons to the central atom.
    Once you get into the 3p subshell and beyond, you can fill the 3d orbital and that will defy the octect. Try PF5. Just add up the electrons, form the bonds, give the outer atoms their octect, and then throw any extra electrons onto the central atom.
    The octect is more a matter of the stability that comes from having a noble gas configuration. I apologize if any of my information is a bit off, I am not an expert. I hope that this helps.
     
  4. Jul 29, 2011 #3
    The octet rule is a rule of thumb that is only really applicable when dealing with the lighter elements (if one ever takes a class on organometallic chemistry, prepare yourself for the 18-electron rule - and of course all of its exceptions). The typical explanation is that an outer shell s2p6 electron configuration is energetically favorable (throw in a d10 when dealing with transition metals).
     
  5. Jul 30, 2011 #4
    Thanks, guys, perhaps I worded my question... What I want to find out is why the octet is more stable than say a 2s22p4 electron configuration. Maybe I will have to get to a more advanced level to understand this, but I always like to fully understand why something happens rather than just accepting it as a rule.
     
  6. Jul 30, 2011 #5
    I'm quite tired right now and about to fall asleep but I see that you are online. If you draw an orbital diagram for Oxygen, you will see that it has two unpaired electrons, when these are filled, by say, Magnesium, then the molecule acquires a noble gas configuration, and that is more stable and less reactive because all the electron shells are filled. I see that you're in high school, are you familiar with and comfortable with orbital diagrams? If so drawing out the molecules and seeing how they are formed by electrons occupying space in other orbitals, then you might see why they are more stable in that state. If you're not comfortable with orbital diagrams, I'm sure I or somebody else will explain them.
     
  7. Jul 30, 2011 #6
    I'll let you go to sleep, lol. I will look it up. I am familiar with the shapes (and understand some of how and why they form in those ways) of orbitals, and I get electron dot notation, I'm not sure what orbital notation is though... maybe I've read it by a different name. Anyways I will look it up so don't worry haha Good night!
     
  8. Jul 30, 2011 #7
    To understand the reason why the octet is considered more stable you have to understand what binds electrons to atoms in the first place, which is the attraction between the negatively charged electron, and the positively charged nucleus. You can think of electrons in lower shells being more strongly attracted to the nucleus, because they are closer to it. The octet rule works well for elements in the second and third row of the periodic table, because for these elements there can be a maximum of 8 electrons in their shells.

    Consider Oxygen, which as a neutral atom has six electrons in it's outermost (valence) shell. Since this is an electronegative element, it tends to gain electrons whenever it's in an ionic bond. How many electrons? Well, we can give oxygen two extra electrons before filling it's shell. If we try to give it three extra electrons, the third one has to go into the next highest shell, which isn't strongly attracted to the nucleus. Since the attraction of the third electron would be so weak, usually oxygen only pulls two electrons, and "forms an octet".

    Consider Magnesium, which as a neutral atom has two electrons in it's outermost (valence) shell. Mg, being a metal, tends to give up it's electrons in an ionic bond. How many electrons? The two electrons in the outermost shell are pretty weakly bound, so they can be given up easily. If you try to take a third electron, it must come from the next lower shell, which is bound much more tightly. So, Mg usually only gives up two electrons, and also "forms an octet".
     
  9. Jul 30, 2011 #8
    Ok so electronegativity is the only reason for octet formation?
     
  10. Jul 30, 2011 #9
    Well ... electronegativity influences whether an atom gives up or gains electrons in an ionic bond. At least for ionic bonding, the whole story is how many electrons will be energetically favourable for an atom to gain or lose.

    In covalent bonding, the simple explanation is a little different. In this case, we can say that it is energetically favourable to form "covalent bonds" which happen when orbitals on each atom, which contain only a single electron, overlap. The lowest energy is when atoms form as many bonds as possible, which, for the 2nd and 3rd row always means the number of electrons they "own" plus the number they "share" adds to eight. Example: Neutral oxygen has 6 electrons (4 paired, 2 unpaired) in it's valence shell. It wants to form two bonds (e.g. H2O, water) by sharing two electrons with other atoms. This means it will "own" 6 electrons and "share" 2 from other atoms, for a total of 8. The numbers just work out that way.

    The origin of the octet rule is actually much much easier to see if you know more complicated bonding theories (like Molecular Orbital Theory), but you don't learn that until university. The main message is there isn't something magical about the octet. The octet rule actually doesn't work well for elements below the third row, for example.
     
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