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Question about the atomic structure of insulators

  1. May 2, 2014 #1
    I'm an apprentice electrician taking night courses. My question isn't a specific mathematical problem. It's a query about a concept. Please let me know if there's a better way/place I can ask it.

    My text say that atoms can have a max of 32 electrons per shell, with 1-3 being ideal for conductors, and 7 or 8 being ideal for insulators. My understanding is that when a valence electron is bumped from its shell, if there is only one electron in that shell, it will take on all of the energy. if there are two electrons in that shell, each will take on half. If an insulator has eight electrons in the outer shell, the energy will be split eight ways. Please tell me if this understanding is correct.

    My questions:

    Why are 7 or 8 ideal numbers for an insulator? If you had 20 electrons in the outer shell, and the energy was divided into twentieths, wouldn't that be a better insulator? Why isn't the best insulator an atom that has 32 electrons in its outer shell, thereby dividing the energy 32 ways?

  2. jcsd
  3. May 2, 2014 #2


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    Staff: Mentor

    That is not true. Shell n has space for 2n2 electrons. In the ground state (!), for all natural atoms and all atoms produced so far, the actual number does not exceed 32, where element 126 could be an atom to exceed this number - but so far, we just discovered the elements up to 118.

    Where does that come from? Atoms with a small number of outer electrons tend to need lower energies to free an electron, but that depends on the binding structure in solids. Metallic bonds usually give a good conductivity, covalent and ionic bonds give a bad one (as they do not have free electrons).
    I guess that statement just refers to s and p orbitals - and then you can have a maximum of 8 in the outermost shell.

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