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Electron Pairs, Outer Shells and Covalent Sharing

  1. Jun 28, 2004 #1
    Four questions for you to answer :-)

    1) Electrons are arranged in pairs ( hence the electron pair repulsion theory) - why is this the case?

    2) Why do all atoms try to fill their outer shells? Why are they most stable if their outer shells are full?

    3) How exactly do atoms "share" electrons in a covalent bond? If the electrons are wizzing round nuclei how are they shared between the two atoms? Is it just a case of them spending some time around one atom and some around the other or is it something more complecated?

    Thanks in advance. :-)
  2. jcsd
  3. Jun 29, 2004 #2
    Can anybody answer any of these please? :-)
  4. Jun 29, 2004 #3
    Dunno what use I will be but I will try.

    Electrons come in pairs because in the orbital series you must have one up spin electron and then a down spin in an orbital (e.g. 1s for helium). Then in Lithium you go to the 2s state and you start with a spin up and then a spin down. You can never have more than a difference of one between the number of spin up and spin down electrons.

    Atoms are more stable with complete outer shells because the number of spin up and down electrons are equal within their orbitals (i.e. the orbitals are complete).

    You have fallen into the common trap here. Electrons are said to spin around the nucleus (like the Earth around the Sun) in GCSE for ease of teaching. In reality they do not. It is simply a consept. I think the covelant bonding is more of the atoms are try to gain another atoms electron (say two chlorine atoms) and they can't take one away and be farther away to completion so they bond with a covalent bond. That way they both get what the atoms want.

    Hope this helps but I am only going on what I have learned by myself (and you cannot learn much in 15 years of existence because you have to learn to talk and walk first (LOL)). :biggrin:

    The Bob (2004 ©)

    P.S. If I am wrong can someone tell me so I know as well please?
    Last edited: Jun 29, 2004
  5. Jun 29, 2004 #4
    I'll do what i can though i'm merely an A level student, i have no degree as of yet, unlike the numerous genius' around here.

    1) Electrons are arranged in pairs as you know, to reduce repullsion via spin pairing. A certain number of electrons can fit in each 'quanta', if electrons span in the same direction yet more spaced out they would repel more strongly and be more unstable.

    2) Atoms tend to 'try to' gain full outer shells as this is their most stable state. The attraction from the nucleus is 'spread out' over the outer shell electrons. as far as i see it. If the energy is spread out over the maximum number of electrons in the outer shell attraction is even and sufficient so no further electrons are lost or gained.
    It seems that if only one/two electron is in the outer shell greater repullsion occurs from the other electrons and thus the electron is easily lost.

    3) Electrons are 'shared' in a covalent bond because they are attracted by the forces of the two nuclei. Between the two atoms the positive forces are greatest as they are being attracted by both atoms, hence electrons are most likely to spend their time there.
  6. Jun 29, 2004 #5
    Don't worry about it. I try and I have just finished GCSEs!!!!!!!!!

    Has it not got more to do with the orbitals of the elements? This would make more sense really. More of the fact that you must have an up and a down spin electron before you go to another down spin electron?

    I can't agree with you (although I am the underdog). The amount of electrons in the outer shell does determine the strength of the nucleui's attraction. It is the number of shells in between the nucleus and the outer shell. The energy spread out doesn't matter otherwise all atoms would have 2 electrons in their whole body.

    Well why do the electrons in the original atom not attract the positive nucleus?

    The Bob (2004 ©)
  7. Jun 29, 2004 #6
    Actually in an orbital it is filled as far as it will go with unpaired electrons before spin pairing occurs.
    Electrons 'want' to be as close to the nucleus as possible despite the fact they only exist in fixed 'quanta', thus they will preferably spin pair before the next orbital begins filling.

    The number of shells in between the nucelus and the outer shell causes shielding and repullsion to take place, so yes, that does have an effect and the greater the number of shells between, the easier it is to lose electrons.
    However, disregarding the number of shells it is arguably easier to ionise an atom with only one electron in its outer shell than an atom with a full complment of electrons in its outer shell. Surely this suggests that repullsion is not having as great an effect on the individual electrons when there is a full shell.

    I'm not sure I understand what you\re asking. There is always an attraction between electrons and protons, but the amount of shielding and repullsion as well as mass/charge ratio effects the first ionisation energies of the elements, as you notice going from groups two to three the ionisation energy becomes easier, against the trend - due to repullsion.

    The Bob (2004 ©)[/QUOTE]
  8. Jun 29, 2004 #7
    Oh I see. I thought in (say) a 2p state you had to fill on orbital ('Cloud') before moving on to another. So you fill all the up spins before starting the down spins?

    Well it will lose or gain the electrons when it wants to. I think the ionsing will be faster, not easier. Again tell me if I am wrong.

    You said that the attraction of the nucleus in one atoms attracts the electrons in another atom. I am saying why do they not do this attraction in the atoms itself not to each other? Or have I missed the point?

    The Bob (2004 ©)
  9. Jun 29, 2004 #8
    Yes, that's correct.

    I don't think time is much of a factor. If the electron is more easily lost then the first ionisation energy is lower, as less energy is needed to remove it. Ionisation energies increase going across the table due to increasing proton pull (though have slight decreases against the trend going from group two to three and from five to six due to repullsion/shielding) and decrease going down the groups due to the fact the outer electrons are significantly further out from the nucleus.

    The nucleus attracts electrons in both the atom it is in and in a weaker way on nearby atoms (pi bonds i think they are called) while electrons repel in both the atom they are part of and other nearby molecules.

    Time to turn this off and go listen to my douglas adams audiobooks >_<, i'll reply in the morning. Feel free to list any questions or correct me anyone.

    The Bob (2004 ©)[/QUOTE]
  10. Jun 29, 2004 #9
    Right. Now understand orbitals. No need to go to that part of chemistry for AS then. :biggrin:

    Ok so time isn't a factor but this energy you talk of is only lower with a lower element (e.g. Lithium to Francium or obviuosly Sodium). Can you explain why the ionisation increases as you go across the periodic table? Cheers :biggrin:

    Right so the strength of the charge in the proton is stronger either because the mass of the electron is smaller or because the charge of the proton is higher. Otherwise they would be equal and the atoms would never meet. Or is it to do with the energy needed?

    Cheers. :biggrin: You are a big help.

    The Bob (2004 ©)
  11. Jun 29, 2004 #10


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    1. Electrons have spins (up or down / +1 or -1). It is found that if you have a buch of electrons, the total energy of the electrons is minimum when the elctrons are coupled in + - pairs. This is a quantum mechanical effect and cannot be explained classically.

    2. Besides spin, electrons also possess other properties like (spin and orbital) angular momentum. These set of properties are charaqcterized by what are known as the Quantum Numbers of the electron. The maximum number of electrons in each shell is determined by these numbers. The fully filled shells are most stable because the minimize the total energy of the atom.

    3. You are exactly right on both counts. It is helpful to think of the electrons spending time around each atom...but that's not really what happens. It IS a lot more complicated.
  12. Jun 30, 2004 #11
    The first ionisation energy (energy needed to remove the first electron in the outer shell) increases going across the table (left to right) due to the increasing proton charge while the electrons are in similar 'quanta' - ie they don't get much further apart from the nucleus.
    On the contrary, when going down, ionisation energy decreases as, though proton charge increases, the outer electrons are also much further out from the nucleus and shielding/repullsion by the other electrons makes it easy to lose the outer ones.

    The strength of charge in one proton isn't greater than that in an electron, but the protons are clustered, attracting the electrons more strongly.

    Hope i answered some of your questions, chemistry (specifically organic) is my bogey subject at the minute so i'm try to help myself too by talking about it.
  13. Jun 30, 2004 #12
    Is this to say that the energy (like KnowledgeIsPower said) is spread out over all of the electrons? Would make sense now that I have read otehr posts.

    Is this to say that the old view of the atom is back or is it simply an easier way of getting out of explaining the complexites? :biggrin: Would like to know complicated side please. Cheers.

    I think my problem here was that I didn't understand what a quanta was. Cheers.

    Duh The Bob (<---me telling myself off). Well this is obvious. I can't believe I had to ask to think (or not as the case is) of it. Cheers again.

    Thanks for you help. I know this was Cheman's Thread but I have found a place in these forums were I feel at home (rather than all of that physics stuff :yuck: (LOL) (If any phyicist comes across this I am joking. I do like physics). Glad that all of my original points were taken and ripped apart and put back together.

    The Bob (2004 ©)
  14. Jun 30, 2004 #13


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    The "spending time approach" is more than just an easy way of avoiding the complexities. It is also leads to fairly accurate predictions of bond strengths.

    The real physics behind the phenomenon lies in that is known as the wavefunction of the electron. All particles exhibit wave-like behavior (see Young's Experiment, etc.) and hence are described by what is called a wavefunction. The square of the wavefunction is the probability distribution of the particle. When you get down to microscopic particles, you can not jut say where it is. But you can tell the odds that it is at a certain place. The charge distribution is a product of the probability distribution and a charge density. The shape of this charge/probability distribution is what is described in chemistry texts as the shape of the orbitals. The chemical bonds are simply electrostatic forces between two charge distributions.
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