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Homework Help: Moving, electron shells?

  1. Apr 21, 2008 #1
    1. The problem statement, all variables and given/known data
    Recently in my chemistry class my teacher told us that, the greater the atomic number the smaller the radius of the electron. Because there is more protons pulling on electrons. which would mean the elctrons move inwards.
    This doesnt seem quite right to me as i thought that the distance of electron shells are constant for all atoms, but i cant find any disproofs or paradoxis with this statment so, am I wrong?

    2. Relevant equations
    none, however an electron shell diagram could be usefull

    3. The attempt at a solution

    i was thinking that if electron shells could move then, theoreticly the atom could collapse by the electrons flying into the nucleus. However this doesnt seem to help me or make sense.
  2. jcsd
  3. Apr 21, 2008 #2
    You're right, it wouldn't make sense. One of the nice benefits of having an "electron cloud" is that the electrons cannot just fly into the nucleus. An electron may only occupy orbitals corresponding to discrete amounts of energy. Because of this, electrons do not tend to collapse into the nucleus.

    Also, every atomic element will have a minimum energy level for it's electrons, but that does not mean that the minimum energy for Hydrogen will be the same for, say, Gold or Lead.
    Last edited: Apr 21, 2008
  4. Apr 22, 2008 #3


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    Hi gateman234,

    The size of the atoms decreases as you move along a single row in the periodic chart. So for the atoms in the third row, with n=3, sodium has the largest radius. Then as Z increases (but the extra electrons keep going into the n=3 shell), the radius decreases until argon has the smallest size.

    But when you go to the next row, potassium, with an electron in the n=4 shell, is larger than all of the elements in the previous row.

    (At higher Z values, the progression isn't quite so regular, but the overall trend still holds.)
  5. Apr 24, 2008 #4
    thanks for this,
    but i still dont really get it
    because wouldnt Na would have energy levels 1,2 complete with one electron in the third shell (3s). while Cl would have 1,2 (3s) energy levels complete with one electron missing from (3p). so Cl would have a bigger radius? unless the energy levels distance from the nucleus changes dependant on the amount of protons. also wouoldnt alot of elements in between Na and Cl have the same radius?

    i must be missing something, help
  6. Apr 24, 2008 #5


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    I think this is correct. As the positive charge in the nucleus increases, the general trend is for the electron orbitals to be pulled inwards.

    For the particular case you mentioned, Cl is quite a bit smaller than Na.
  7. Apr 24, 2008 #6
    the whole point of classifying subshells as for example, s or p, is because the geometric arrangement is different. S are basically spherical, wheras p are dumbbell shaped. Hence, an s electron can be on average the same distance from the nucleus as a p electron. Just because an electron is in a p orbital does NOT mean that it must be further away. As effective positive nuclear charge increases (i.e. you move along a group), ALL the electrons are "pulled" further in hence decreasing the atomic radius
  8. Apr 24, 2008 #7

    what about Bores model?
    because doesnt the different geometric arrangments (s,p,d,f)
    fill in that order?
    also, because there are more protons there are also more electrons. so there isnt any exess charge. however if we were talking about ions, it would be different.
    i think?
  9. Apr 24, 2008 #8


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    Bohr's model also predicts that atoms become smaller as you move to the right along a line of the periodic chart. (In his model the electrons did not interacts with other electrons in the same shell.) So in his model:

    Start with hydrogen with a specific radius. Then go to helium. Each electron "sees" a nuclear charge of +2 and is pulled in closer to the nucleus.

    Lithium: each of the n=1 electrons "sees" a charge of +3 and is pulled even closer to the nucleus. For the third electron in the n=2 orbital, it experiences a nuclear charge of +3 but the screening due to the inner electrons create an effective central charge of +1 and so, based on this model, we would expect the third electron to be roughly at the location of the n=1 shell for hydrogen.

    (However, the reasoning here is just the reasoning in Bohr's model; in real atoms the electrons all interact even within a shell, and so this simple model does not give good numerical results.)

    I was only referring to neutral atoms. If instead you consider, for example, hydrogenic ions (with one electron) then size will steadily decrease as you move to higher Z. Also, if you only ask for the size of the 1s shell in a neutral atom, that will also steadily decrease with increasing Z.
  10. Apr 25, 2008 #9
    Yes that's true, they do fill in that order, but that's not because the subshells are "further out", from the nucleus. We always talk about subshells being something that is actually real, something that is already there that an electron can just drop into, but in reality it's just the electrons natural way of orbiting an atom that we call a "subshell". There are good quantum mechanical reasons why electrons naturally follow these shapes, and why the s subshell has a lower energy than the p, eventhough they are on average the same distance from the nucleus (in rough terms). It's all explainedhttp://en.wikipedia.org/wiki/Shell_model" [Broken], but without meaning to sound patronising, I think it's a little more advanced than your current level of chemistry. It's certainly more advanced than mine!
    Last edited by a moderator: May 3, 2017
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