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Why do atoms want a full outer shell?

  1. Nov 3, 2003 #1
    Why do atoms want a full outer shell
  2. jcsd
  3. Nov 3, 2003 #2

    the answer for your question is simple.
    To attain stable electronic configuration they they try to have full the outermost orbit or valence orbit.This is done by losing or gaining electrons.
    Last edited: Nov 3, 2003
  4. Nov 3, 2003 #3
    Re: answer.........

    Isn't this more of a description of what happens, rather than an explanation? I would now like to give a far better answer...... but sadly I can't think of one!
  5. Nov 3, 2003 #4

    Does anyone know why, is there a force involved, or an equalibrium that is trying to be acheived, if anyone has any idea please say
  6. Nov 3, 2003 #5

    Chi Meson

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    I'm with Adrian Baker on this one.

    The answer is either too simple or too complicated. It's too simple to say that "it's a quantum rule that must be followed." And it's too complicated to describe those rules.

    I'm eager to read what follows!
  7. Nov 3, 2003 #6


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    It is quantum mechanical. There are eigenstates for the electron configurations of atoms. Some neutral atoms have deficit or surplus m and/or s numbers. To take a simple example, consider Hydrogen. It has a stable electron configuration with two electrons, This represents the two possible angular momentum states of the ground state. One of the spins must be up and the other must be down. This gives an attractive spin-spin interaction, and thus, Hydrogen forms diatomic molecules.
  8. Nov 3, 2003 #7
    sorry can u explain to me what spin is please. Its more complecated than just simply a turning motion isnt it? what does it mean when spin = half.

    Is spin a known quantity which can be derrived by base quantities?
  9. Nov 3, 2003 #8

    Chi Meson

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    It's going to go on like this for a while. THings will get more complicated before they get easier. But no one is answering your question yet, so...

    As Turin stated, there's this thing called spin and it's part of what explains why atoms "want" a full valence shell. Spin is not "spinning." When they called it "spin" in the first place, they just needed a word and it stuck. Spin is another characteristic of matter, similar to mass and charge being a characteristic of matter; these quantities are not made up of combinations of other quantities.

    All subatomic particles have a spin quantum number. Electrons have either a + or - 1/2 spin. Half-integer spin is a characteristic of those particles that we think of as "material," "stuff," etc. neutrons and protons also have 1/2 interger spins. THis stuff obeys a certain set of quantum rules.

    Full integer spin particles like photons obey a different set of quantum rules. SOme particles have "zero" spin, which is not to say they are "without spin," but just that they have a "full integer spin" number between -1 and +1.

    BTW, I welcome any correction to this. I'm trying to keep things as basic as possible without being oversimplified.
  10. Nov 3, 2003 #9
    My understanding is that the label +- 1/2 is just to remind us that there are only two possibilities. They are often called up and down and could have been called left and right but +- 1/2 stuck.
  11. Nov 3, 2003 #10


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    The 1/2 and -1/2 are more than just names.

    The description of quantum states is done via vectors (in not-so-simple spaces). Those vectors can be transformed by objects called "operators", and there is one operator for every quantity we can measure.

    The relation between nature and the formalism is such that the values that can be obtained in a measurement are always "eigenvalues" of the corresponding operator.

    1/2 and -1/2 are the eigenvalues of the operator that, in the formalism, represents spin.

    Also, they are the only possibilities only for particles with a total spin equals to 1/2. There are systems with a total spin equals to 3/2, and for them there can be spin values of -3/2, -1/2, +1/2 and 3/2.
  12. Nov 3, 2003 #11
    The reason why a full shell is more stable is more or less the same reason a half-filled shell is stable. There are a set of guidelines called Hund's Rules which are used to determine the ground state electron configuration of a particular atom. The underlying principle is that electrons fill orbitals in such a way as to minimize the total energy of the atom.

    Hund's 1st rule says that for a particular system of electrons, the one with the largest total spin is the lowest energy. That is, the atom wants to have the largest number of parallel spins that it can (for example, two spin up) as opposed to a paired up and down spin. The reason for this is as follows: two spins that are parallel "don't like each other", so they will tend to push apart. This decreases the Coulomb energy between them, lowering the overall energy of the atom (this is due to something Pauli Exlusion).

    Now, let's pretend we are in a particular subshell of the atom, say the 3d one. This is a labeling system used often in quantum mechanics. All we really need to know is that 3d has 5 possible orbitals, each of which can hold 2 electrons (one spin up, one spin down). In order to get the largest number of unpaired spins, how do we fill it? We take 5 electrons, and place one in each orbital, giving us 5 unpaired spins, for a total spin of 5(1/2)=5/2. If we add another electron, it will pair up with and cancel one of these, leaving 4 unpaired spins, for a total spin of 4(1/2)=2. Larger spin means less energy, so we see that the 1st case is more desirable; that is, a half filled shell is generally more stable than you might think. Similar considerations can be applied to a full shell.

    Of course, to fully detemine the electron configuration, we need to apply the rest of Hund's rules (2 of them), which are more complicated, and break down for certain elements.
  13. Nov 4, 2003 #12


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    (1) A half filled shell is stable? I guess you could say it's stable, in the same sense that every nonradioactive atom has a stable configuration, regardless of how full or empty the valence shell is. Iron "turns into" rust because it prefers a "full" valence shell, regardless of how "stable" it is as atomic iron. I think the question is more, "why do atoms want to fill their shells the way they do?" not, "why is a full valence shell a stable configuration?"

    (2) Guidelines don't help most people understand; they just organize phenomena that we don't understand in a way that is easier to deal with, so that we don't have to memorize every possible configuration.

    (3) Minimization of total energy is not always the driving motivation of phenomena. How do you explain stable helium with this reasoning? I could be wrong, but I thought that stable Helium in the ground state had one up and one down spin electron in the s1 shell, as opposed to two up or two down.

    (4) That's why they are called "guidelines." They don't explain, or even describe, what's going on; they just organize phenomena into a more concise group.
    Last edited: Nov 4, 2003
  14. Nov 4, 2003 #13
    1) By stable, I simply meant it is generally more stable than certain other configurations, say half filled plus 1.

    3) Yes, you are right. The 1s subshell (n=1, l=0) can hold two electrons. Once the first one goes in, the second has no choice but to go in antiparallel to the first. Energy is still minimized, even though the spins aren't parallel (usually, electrons will fill one subshell before moving onto another, at least in this case).

    4) Of course, taken at face value Hund's rules don't explain "why" stuff works, they only provide a convenient way to determine electron configurations. To really understand the process, you have to look at the physical motivation behind them. By the way, all 3 rules are based on minimization of energy: the 1st two with the energy due to Coulomb interactions, and the 3rd with spin-orbit coupling. It is the 3rd rule that breaks down for certain elements (specifically, 3d ions), in which something called crystal field interactions become more important than contributions from spin-orbit coupling.
    Last edited: Nov 4, 2003
  15. Nov 5, 2003 #14


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    (1) Alright, I'll buy that.

    (2) I don't see this so much as minimization of energy. It seems more like Pauli exclusion: The spins are antiparallel because they can't be parallel, since they are fermions. I can see why it would look like minimization of energy. If the spins were parallel, and in the same shell, then that shell could not possibly be the 1s shell, so it would have to be a higher energy orbital. Ya, OK, I think I see what you're saying.

    (3) My point was that, minimization of energy alone doesn't have anything to say about the fermionic behavior, as I understand it. The bound states of the electrons in an atom choose the particular configuration that they do because they are fermions, and the configurations are eigenstates of some electrostatic potential and some spin coupling. This can dictate a minimization of energy, but the minimization of energy is not a necessary feature of the QM rules that govern the configuration. Correct me if I'm wrong.
  16. Nov 5, 2003 #15
    (1) :smile:

    (2) Pauli Exclusion leads to minimization of energy here. Because of the PEP, parallel electrons don't like each other, and will tend to push apart, reducing the Coulomb energy between them. Or if you like, fermions require an antisymmetric wave function. Parallel spins are symmetric, so the spatial component must be antisymmetric - the electrons will thus be farther apart on average. And like you said, He only has 2 choices - antiparallel in 1s, or jump to a higher orbital.

    (3) For two random fermions in some system, PEP governs their behaviour, and energy does not necessarily have anything to do with it - no two fermions can share the same quantum state, end of story. If we start pumping electrons into shells in an atom, they will fill up in such as a way as to minimize the total energy. PEP plays a role, as does angular momentum and spin-orbit coupling.

    Whether or not min. of energy is the only force behind the electron's behavior, I guess I honestly don't know. All of my courses listed it as such :wink:.
  17. Nov 6, 2003 #16


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    Not to nitpick (sp?), but "Minimization of energy" is never a driving force. In this case there are mainly electromagnetic forces, which have to play in the stage set by the quantum rules. We call "energy" a number that is defined to be minimum when nothing further can be extracted from the system. If the system is such that it spontaneously "goes" to a different state, then we were not in the minimum yet.

    You probably knew this already, but it may be useful for other people.
  18. Nov 6, 2003 #17
    There is an alternative way of explaining what is happening that goes as follows.

    Note that each addition of an electron causes a decrease in the atomic radii except where the additional electron(s) complete an electron shell; when the addition causes an increase in atomic radii.
    At lower densities only one electron is needed to complete the shell, at medium density two electrons are required, finally at high density three electrons are needed. That is to say that during shell completion, the atomic radius increases in one, two or three stages depending on mass or density.
    It should therefore be possible to create a table of contraction and expansion based solely on density, but unfortunately the available data is based on average measurements which lack the accuracy required to construct such a table.
    The cause of this contraction and expansion can be attributed to magnetic force only if one creates two magnetic forces (i.e. attractive and repulsive). This is what current theory does. Just don,t ask what magnetism is!.
  19. Nov 9, 2003 #18
    I need to ask. Is there really such a thing as and unbalanced atom? If there is, is this the reason for "Brownian Motion"?
  20. Nov 12, 2003 #19
    Atoms are assemblies of particles and therefore do not have a smooth surface (field boundary layer). Surface imperfections and spin are the cause of Brownian motion.
  21. Nov 12, 2003 #20


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    Brownian motion has nothing to do with the electronic configuration of the atom. Brownian motion results from collisions between atoms and small particals, this is a macroscopic, visiable, phenomena electron shell configuration is on a significantly smaller scale.
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