Why is there sudden increase in atomic radii when moving from neon to sodium?

Discussion in 'Chemistry' started by ash64449, Jun 1, 2013.

  1. exchemist Valued Senior Member

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    It's interesting. I'm being a bit circumspect, because my old Cotton & Wilkinson (3rd ed.) says as follows (p.535: "It is also well to point out that the interelectronic forces and variations in total nuclear charge play a large part in determining the configurations of ions. We cannot say that because 4s orbitals became occupied before 3d orbitals they are always more stable. If this were so, then we should expect the elements of the 1st transition series to ionise by loss of 3d electrons, whereas in fact they ionise by loss of 4s electrons first. Thus it is the net effect of all forces -- nuclear-electronic attraction, shielding of one electron by others, interelectronic repulsions and the exchange forces -- that determines the stability of an electronic configuration; and, unfortunately, there are many cases in which the interplay of these forces and their sensitivity to changes in nuclear charge cannot be simply described."

    So yes indeed it seems it is the 4s that get stripped off first. I suppose this makes sense in that taking out a 4s reduces the shielding of the 3d, which then drop in energy, leading to an overall lower energy ground state than you would get by taking out a 3d electron and leaving the 4s in situ.

    But in general my recollection of the transition elements is that the ability to predict their chemistry from their electronic configuration becomes rather weak, compared with what you can do in the s and p block. There are just too many interacting factors.
     
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  3. exchemist Valued Senior Member

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    I'm not sure I would read too much into this. Take a look at the attached, about the alternative meanings of "atomic radius": http://en.wikipedia.org/wiki/Atomic_radius

    The thing is that since the electron cloud tails off exponentially with distance from the nucleus, any "radius" one may assign is a bit arbitrary. All one can measure is a mean value, from various interatomic distances in a range of compounds, or some kind of theoretical radius. I suppose one would expect the atomic radii of Cr and Mn to be very similar, as the only difference is that nuclear charge increase by 1, while one extra electron is paired in the 4s subshell.
     
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  5. exchemist Valued Senior Member

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    Ash,

    1) 3d5 4s1 IS the subshell configuration. The principal shells are 1, 2, 3 etc and the subshells are s, p, d, f....

    2) The link I tried to send you seems to send you a pdf file. I tried it myself just now and it sent me another copy, so seems to be OK for me at least. But I found it by googling "half-filled shell stability". Perhaps you can try this. The pdf comes from Canterbury University in New Zealand.

    3) If the energy of an excited state configuration were not close to the ground state, I would expect it to occupy more space, i.e. so that the electrons have more potential energy, due to being on average further from the nucleus. (As you go to higher and higher excited states you get closer to the ionisation limit, where the electron is detached entirely.)
     
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  7. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    @Trippy
    What's your opinion on the matter?
     
  8. ash64449 Registered Senior Member

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    795
    i referred by textbook and i understood that the word should be called as 'degenerate orbitals'. I meant that one.
     
  9. ash64449 Registered Senior Member

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    795
    I didn't understand this point correctly.

    Yeah.. I think that is why they are called representative elements(s and p i mean). They show correct periodicity.

    d-block and f-block are much complicated..
     
  10. arauca Banned Banned

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    4,564
    I believe the radii should be the same . Think if you are in the S shell one electron spins in one direction and the second will spin in the opposite direction so they will be in the same plain
     
  11. exchemist Valued Senior Member

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    Ah, OK. "Degenerate" orbitals simply means a set of orbitals with identical energy. (I think "degeneracy" used in this sense is a term inherited from spectroscopy, whereby 2 or more potential spectral lines lie on top of one another and cannot be seen as separate.) In the case of the d orbitals there are 5 in a free atom. As all 5 are identical it is meaningless to try to say WHICH of the 5, say 2, 3, or 4 electrons are in, as there is nothing to define a system of coordinates that would distinguish them. However if the symmetry of the environment is reduced in some way, e.g. by an electric or magnetic field (defining a "z' axis in space), or by the atom being in a crystal or a molecule with a geometric pattern of other atoms around it (defining "x", "y" and "z" axes in space), then indeed the various d orbitals will in general no longer all have the same energy and the electrons will take up particular orbitals with distinct spatial distributions with respect to the field experienced by the atom. The degeneracy is "resolved" and the spectral lines "split", revealing the individual energy levels represented by each orbital, resulting from its orientation with respect to the field.

    This however is a big subject, covering both Zeeman and Stark effects in atomic spectra, and Crystal Field Theory (in ionic complexes and crystal lattices), and Ligand Field Theory in complexes with more covalency. You can look these up if you are interested, but it will be quite a long read!
     
  12. ash64449 Registered Senior Member

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    795
    One Thing that i am not able to understand is that why d-block elements don't show correct periodicity as that of Representative elements?

    Same case for f-block elements.

    Can someone help me find me an article or any book reference so that i can understand their weird periodicity?
     
  13. exchemist Valued Senior Member

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    6,478
    I'm afraid some of this you just have to learn. We've already discussed the complex range of interacting factors that make it hard to predict detailed chemical properties from electronic configuration in the d block. There are some trends that derive from the configuration, such as the range of oxidation states, the "lanthanide contraction" and "actinide contraction", some magnetic properties and so on. And there is a case for treating the last column of the d block as not really "transition" elements at all, due to the filled electronic configuration which changes the way they behave, compared to the others. But a brief glance at - for instance - the oxo- complexes of molybdenum can be enough to make you pull your hair out! As to books, I'll be very out of date by now. I confess that at university I relied heavily on Cotton and Wilkinson and did not explore the d or f block elements in enough detail to warrant getting books on them specifically.

    I think that in a way, for these elements, this IS the "correct" periodicity, if you like. The overall conclusion is that the filling of the d orbitals leads to a large number of elements with confusingly similar general behaviour, and you have to zoom in at a level of greater detail to grasp the intricate differences. But I'll happily bow to the greater knowledge of a real expert on these elements.
     
  14. ash64449 Registered Senior Member

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    795
    Yeah!!! It is 'correct' periodicity!! I just wanted to mention why they were different. That's all!!
     
  15. ash64449 Registered Senior Member

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    795
    So Exchemist,

    I am currently reading a chapter called classification of elements and periodicity in properties in my textbook and it was discussing this trend i.e atomic radii, So for more information,i took information of Wikipedia about atomic radii of all the elements and saw many exceptions. I was able to get answers of like why the graph of atomic radii vs atomic number is not straight and all .. But i couldn't answer the d-block periodicity.

    So i think i should leave it and focus on representative elements.
     
  16. exchemist Valued Senior Member

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    Yes I think that's wise. There are many explanations of transition element properties, e.g why Hg is a liquid, due to the lanthanide contraction and its filled d and f subshells, but to me these always have the look of post hoc rationalisation, i.e. if the property were different one could just as easily have come up with an explanation for that instead! Whereas there is no doubt that for the s and p block elements a lot of things fall into place when one considers the electronic configuration - especially for the lighter elements, i.e before the d and f block get to work on obscuring everything!
     
  17. wellwisher Banned Banned

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    The answer to why there is an increase in atomic radii from neon to sodium has to do with the electromagnetic force (EM). The EM force has both electrostatic and magnetic components. The EM force for the outer electrons is stronger for neon compared to sodium. Both have balance between electrons and protons, so the main source of the EM force difference is connected to the magnetic component.

    A charge in motion will create a magnetic field, therefore orbiting electrons generate magnetic fields. Electron moving in the opposite direction will create attractive magnetic fields, even though there will be electrostatic repulsion between two like charges. The magnetic is stronger due to the fractional light high speed of the electrons.

    In the case of neon, its electrons magnetically add in a very efficient way to due the 3-D nature of the p-orbitals. There are two legs in each of the x,y and z, directions. This strong attraction has to do with the right hand rule for charge in motion, with the three perpendicular x,y,z p-orbitals allowing each electron motion to perfectly align their right hand rule force components with the other two orbital directions (x, y z). It is a perfect way to compile strong magnetic attraction.

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    Sodium has extra electron which does not magnetically integrate very well with the 3-D p-electrons. The result is sodium is large and very reactive and will give easily off the extra electron to return to the core magnetic stability.
     
  18. exchemist Valued Senior Member

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    Wellwisher, something is not right here.

    Successive principal shells (n=1, 2, 3....etc) have higher energy than their predecessors due to the form of the solutions (eigenfunctions, with associated eigenvalues) of Schrodinger's wave equation (time-independent form).

    http://chemwiki.ucdavis.edu/Physica...ple_of_Quantum_Mechanics/Schrödinger_Equation

    This is analogous to a fundamental resonant vibration and its successive harmonics. It is NOT a magnetic phenomenon.

    The atomic spectrum of hydrogen, which shows a sequence of energy levels corresponding to n=1, n=2, n=3, etc., even though only one electron is present, demonstrates that magnetic effects between electrons are not responsible.
     
  19. ash64449 Registered Senior Member

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    795
    I would like to ask a question: What is the difference between nucleus attracting the electron and electronegativity?
     
  20. exchemist Valued Senior Member

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    Ash, electronegativity is a qualitative ranking of elements according to the liking they seem to have for an extra electron in chemical reactions. There's an article here about it: https://en.wikipedia.org/wiki/Electronegativity

    It is not to be confused with the quantitative physical property of Electron Affinity, which is the energy released when an atom captures an extra electron into an unfilled orbital, e.g. Cl + e- -> Cl-. Electronegativity is related to this, but is more broadly applicable, for example reflecting the tendency of an atom to form an anion, the direction and degree of polarisation in a bond between 2 atoms of different elements, and so on.

    Electronegativity and Electron Affinity can be predicted quite well from the sort of considerations we've discussed before: it tends to increase across a period since the increasing nuclear charge pulls down the energy levels of the subshells, so that, provided there is still space in at least one of them to accept an electron, tendency to do so will increase.

    So, yes, it is related to the strength of the net attraction of the nucleus for an extra electron, but in a qualitative way.
     
  21. ash64449 Registered Senior Member

    Messages:
    795
    yes. thank you!!!

    while considering the reactivity,should't we consider the element having least and the highest electronegativity as most reactive? and we should't consider in terms of.ionization enthalpy in metals and electron gain enthalpy in non-metals?
     
  22. exchemist Valued Senior Member

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    Well yes! Try putting sodium metal and chlorine gas together, for example!

    Although "reactivity" is a bit of a loose concept. What we're really talking about is the thermodynamic driver for reaction, i.e. the enthalpy of reaction. We always need to bear in mind also that kinetic factors need to be considered, when discussing how fast and furious a reaction may be. For example, aluminium should oxidise very fast in air, but it doesn't because a thin film of protective oxide on the surface stops the reaction from proceeding. Or, you can keep a mixture of hydrogen and chlorine gas in the dark indefinitely without reaction. But if a bit of UV light gets to it...BANG!!
     

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