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Less electronegative atom is the central atom. $\ce{N}$ is more electronegative than $\ce{Cl}$ as $\ce{Cl}$ has larger atomic radius. So why in $\ce{NCl3}$, $\ce{N}$ is the central atom?

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Cl is slightly more electronegative according to the tables I've seen: http://en.wikipedia.org/wiki/Electronegativity

But I wouldn't say it's an absolute rule that the less electronegative atom is central. For example, $\ce{NH3}$ has the N central and $\ce{Cl_2O}$ has the O central.

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  • $\begingroup$ The relative positions of chlorine and nitrogen on the electronegativity scale can switch depending on the definition of electronegativity being used. Allen scale electronegativities are quite recent but also popular, and there nitrogen is higher than chlorine. But yeah, the asker is just taking a rule of thumb too strictly. $\endgroup$ – Nicolau Saker Neto Jun 14 '14 at 19:37
  • $\begingroup$ Acc. to Pauling's scale, both have electronegativity 3.0 but as $\ce N $ has lesser atomic radius,it is more electronegative.But here it seems contradiction as a lesser electronegative atom ought to be the central atom.But it is also accurate that in nitrogen tri chloride , nitrogen is the central atom .Is the 'less electronegative atom is the central atom ' a wrong concept? $\endgroup$ – user5764 Jun 15 '14 at 1:59
  • $\begingroup$ How can I determine accurately the central atom ? $\endgroup$ – user5764 Jun 15 '14 at 2:01
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    $\begingroup$ @user36790 I would say that the example of Cl and O basically proves there is no simple set of rules, since Cl can be in the center for chlorate, perchlorate and other compounds, while $\ce{Cl_2O}$ has the O central. Also, the symmetrical choice or lone atom in the center, isn't always correct as there is NNO. The rule "H is never in the center" you can usually count on, although a hydride can bridge two metal or boron atoms. I would say that carbon and nitrogen tend to be central slightly more than their electronegativities suggest and halogens non-central. SeCSe, CAt4, NI3. $\endgroup$ – DavePhD Jun 16 '14 at 12:11
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Being "central" has nothing to do with electronegativity. It is a matter of geometric symmetry. The atom at the "center" of a molecule is the atom with the highest geometric symmetry. But if you look at benzene, there is no atom at the "center" of the molecule.

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