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Valence Bond question

  1. Nov 13, 2004 #1
    I'm working a problem with the BH3 molecule. Therefore boron is 1s^2 2s^2 2p^1, and using the hypotetical method, it forms hybird sp2 orbitals. Promoting an electron to get 1s^2 2s^1 2p^2.

    My question is that, it will form 3 bonding orbitals, one from the 1s and 2 from the 2p orbitals... what happends to the third 2p orbital? If I do an energy level diagram, do I have to list this orbital as a nonbonding one?
     
  2. jcsd
  3. Nov 14, 2004 #2

    chem_tr

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    I think you didn't intend to write "1s", but you'd write "2s". In a hybrid orbital like sp2, s orbital is raised to a higher energy while p orbitals' are somewhat lowered, eventually meeting at the same energy level. The third orbital will be unaffected, due to boron's geometric hybrid state. However you may force this hybrid system to give a sp3 hybrid orbital, as in BH4-. What you'll do is basicly leave this 2p orbital unattended (at the same energy level), to show that it is not involved in this type of bonding.
     
  4. Nov 14, 2004 #3
    Therefore boron will have one 1s three sp2 and only one 2p orbital, and when bonding with thy hydrogen it will leave (from bottom to top in energy) a 1s orbital nonbonding, 3 sigma bonds, a nonbonding pz (for example) and three antibonding?
     
  5. Nov 14, 2004 #4

    chem_tr

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    You are trying to apply molecular orbital theory on the compound, in which hybridization is used to describe the phenomenon. If you will use MO theory, then it will be of no use to mention about hybridization, I think. Hybridization leaves its place to MO, that's right. But I couldn't devise a diagram for MO, as it is three-atom-based MO diagram (which I encounter difficulties in understanding).
     
  6. Nov 14, 2004 #5
    Thank you chem_tr, thank you very much for your help.
     
  7. Nov 20, 2004 #6
    Electrons are attracted to protons, but repell electrons. So, instead of all the electrons being bunched up right next to the nucleas, they orbit around the nucleas in shells. These shells can sometimes contain sub-shells. For example, the first shell contains only one sub-shell. As an electron gets further away from it's atom, it must have more "quantum energy." Electrons want to get as close to the nucleas as possible, but according to quantum physics, no to electrons can have the same "quantum energy." So, they orbit in shells. The electrons orbit in orbitals. The sub-shells have orbitals. For example, the 1 shell has an S orbital. Because it's an s orbital and it's the first shell it's labelled 1S. For 1-First shell-, S-S orbital. An S orbital has the shape of a sphere. An orbital wants to fill it's self. Alright, so why would the atom want to have 8 electrons in it's outer most shell, good question. The second shell has two sub-shells. One sub-shell has an S orbital, and the second has three P orbitals. The reason it has three is because they can arrange themselves according to X,Y,Z. Each orbital has only two electrons, because no two electrons can have the same "quantum energy." So, for the valence shell of an atom with two shells, one S orbital and three P orbitals. Two electrons an orbital adds to...8. Hydogen, on the other hand, only has one shell. So, to fill it's valence shell, it only needs two electrons. It already has one - Hydogen = one proton, one electron - so, it only needs to bond with one atom to fill itself. Carbon, on the other hand, has two shells, so it needs 8 to fill it's valence shell. So....

    H
    H C H Methane!!! CH4.
    H

    If you were to count it up everyone's filled. The carbon atom has 6 electrons. 2 in it's first shell, and 4 in it's valence shell. It needs 8 in it's valence shell. So, it shares one with hydrogen, and the hydrogen shares one of the carbons. This gives the carbon an extra electron, and the hydrogen it's desired two. The carbon, then, bonds with three more to add to 8.

    HOH Water!!! H20. Oxygen has six valence electrons, meaning it needs 2 to gain, which it does with 2 hydrogen molecules.

    O=O Oxygen!!! O2.

    You're probably wondering, why is there an equals sign between the Oxygen molecules?
    This indicated a double bond. Oxygen has six valence electrons, when it bonds with another oxygen, it gets 7. That's not the desired 8. So, it makes a double bond, and they share two electrons each. Which adds to 8.

    O
    O O Ozone!!! O3. Each one of these atoms share with each other, making 8.

    That's covelant bonding!!!
    This "quantum energy I told you about is somewhat true. What's really true is that there are four "quantum numbers" that cannot match.
    The first is N.
    N is the energy of an electron. For example, an electron in the first shell would have an N of 1. An electron in the second shell would have an N of 2. An electron in the third shell would have an N of 3.
    N=1, means it's in the first shell.
    The second is L. It's actually a greek cursive L kind of like this. l. Okay. This sign is the orbital. L = N - 1. That's the equasion. So, if N = 1, then, L = 0. 0 is an S orbital.
    If N = 2, L can equal either 0 or 1. If it is 1, that's a P orbital. If N = 3, then that can be either 0,1 or 2. An S,P or...a D orbital.
    Now, the third quantum number is M. It is the orientation of the orbitals, you know XYZ.
    M can equal anything between -L and +L. For example if L is 1, then M can equal -1,0,1.
    This is 3 different ways of arranging the P orbital.
    Now the final one is Ms. For Spin. The spin of the electron can equal - 1/2 or 1/2.

    Okay, so let's look at the possible arrangements of some electrons.

    N L M Ms
    1 0 0 -1/2
    1 0 0 1/2 First shell, only can have two electrons.

    2 0 0 -1/2
    2 0 0 1/2
    2 1 -1 -1/2
    2 1 -1 1/2
    2 1 0 -1/2
    2 1 0 1/2
    2 1 1 -1/2
    2 1 1 1/2 Second shell, eight electrons, but none of them, nor the one's in the first shell have the same 4 quantum numbers.

    HOPE YOU UNDERSTAND. IT TOOK ME A WHILE TO WRITE, I'D HATE TO LOSE IT AT THE LAST MOMENT, LIKE THE POWER SHUT DOWN OR SOMETHING. IF YOU UNDERSTAND THIS, YOU WILL UNDERSTAND THE REST.
    HERE'S SOME SITES.

    http://chemed.chem.purdue.edu/gench...h6/quantum.html

    http://lectureonline.cl.msu.edu/~mm...od/electron.htm
     
  8. Nov 20, 2004 #7
    Hey Dual op Amp, ask moderators to create a sticky post with that! (the fourth time)
    :wink:
     
  9. Nov 21, 2004 #8
    What's a sticky post, and how do I ask a mod?
     
  10. Nov 21, 2004 #9
    Excuse my English, I meant "sticky thread"

    A sticky thread is a tread that remains everytime in the first place of the queue, for example Monique´s "chemical laTEX typeset" thread in this section. Moderators post them as a note for everybody to read them.

    Only moderators can post them, as far as I know you can´t ask them to post a sticky thread. I was just joking, cos you post 4 times the same thing.
    If you want to tell things that you´ve already post you can insert a link to it.

    Anyway that reply reveals effort in explanation and is a good try to help. Congratulations.
     
    Last edited: Nov 21, 2004
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