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Describing bond between elements

  1. Jan 23, 2009 #1
    [​IMG]

    in case A:
    S has a bigger electonegative energy then Ca
    so it will "take" 2 electrons from Ca inorder to be a complete gas like (Ar)

    so i know why there is +2 on Ca and -2 on S .

    Ca has after perfect gas 4S^2
    S =[Ne]3s^2 3P^4
    but there are dots and cols what they mean??

    regarding B:
    for Br to become a perfect gas like(Kr)
    it needs to take only one electron
    why there is -2 ??
    and what are all the dots nere it and cols []??
     
  2. jcsd
  3. Jan 23, 2009 #2

    Borek

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    Staff: Mentor

    Eight dots as octet (eight electrons) on the valence shell.

    [] just groups atom and electrons together, so that when you write [Ca]<sup>2+</sup> or [Ca:]<sup>0+</sup> it is obvious that charge is that of the atom and the electrons together.

    Where do you see -2? There are two Br<sup>-</sup> anions (each with its own octet), that gives -2 in all, but there is no individual entity with this charge in the magnesium bromide.
     
  4. Jan 23, 2009 #3
    on case A
    the formula for S is [Ne]3S^2 3p^4

    from where in this formula i can see that there is 8 electrons on the valence shell??
     
  5. Jan 23, 2009 #4

    Borek

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    For S2- it becomes [Ne]3s23p6 - hence the octet. [Ne]3s23p4 is a neutral S atom.
     
  6. Jan 23, 2009 #5
    ok i understand case A

    regarding case B:
    Mg=[Ne]3s^2 so it has 2 electrons on the
    valence shell before the merging.

    Br=[Ar]3d^10 4s^2 4p^5
    so it has 17 electrons on the
    valence shell before the merging.

    after the merging Br needs to take 1 electron from Mg to become neutral gas.

    and for Mg to become a neutral gas we need to take 2 electrons
    so we link two Br atoms

    but why they present 8 dots around Br??

    before it was 17 then we add 2 so we have 19
    but 2*8=16

    ??
     
  7. Jan 23, 2009 #6

    Borek

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    To get stable octet.

    We don't add two electrons. We add one electron to each of the two Br atoms. You have to treat each atom separately.

    d electrons can be in this case ignored, they don't play any role.
     
    Last edited: Jan 23, 2009
  8. Jan 23, 2009 #7
    regarding case B:
    before Br has 17 electrons
    after the reaction each Br gets 1 electron
    so each Br must have 18 electrons

    but in the solution
    they have only 5
    ??
     
  9. Jan 23, 2009 #8

    Borek

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    Picture is fuzzy, but they have 8, just like sulfur. : on the left, : on the right (it is "Br:", not "Br."), two above and two below.
     
  10. Jan 23, 2009 #9
    ok even if its 8
    before Br has 17 electrons
    after the reaction each Br gets 1 electron
    so each Br must have 18 electrons

    but in the solution
    they have only 8

    its not 18
    ??
     
  11. Jan 24, 2009 #10

    Borek

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    I have already explained: d electrons are ignored, they don't play any role.
     
  12. Jan 24, 2009 #11
    why egnoring
    D

    ??
    3d^10 4s^2 4p^5

    they do play a role
    they all
    are on the
    valence shell
     
  13. Jan 24, 2009 #12

    Borek

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    Octet rule is only a simplification. Obviously it can't work for any atom where d electrons take part in the boding, as there are 10 of them and octet calls for 8 electrons.

    In this particular case s&p electrons are enough to explain what is going on, d electrons are not involved - they sit on their orbitals untouched.
     
  14. Jan 24, 2009 #13
    thanks
    :)
     
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