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How to find the central atom? What is the central atom in $\ce{PCl3}$ and how to find the shape using VSEPR theory?

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    $\begingroup$ You don't really have to look for the central atom. Technically, VSEPR theory may be applied to any atom in the molecule, only in case of chlorines it would not produce anything useful. $\endgroup$ Apr 5 '16 at 21:08
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You know that phosphorus has 5 valence electrons and that chlorine has 7 valence electrons ... and that you somehow have to combine them.

As chlorine is more electronegative (Allred-Rochow: Cl 2.83, P 2.06) you know that each chlorine will attract one electron from the phosphorus to get the noble gas configuration with 8 electrons in the valence shell.

Now that in turn means that there will be one single bond between the phosphorus and each chlorine, which automatically makes phosphorus the center.

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In general the least electronegative atom occupies the central position in the molecule/ion. For example, in PCl3, chlorine is more electronegative than phosphorus, occupy the terminal position and phosphorus is the central atom. Hydrogen is an exception as can NEVER be the central atom. The atom with the lowest subscript in the molecular formula indicate the number of atoms of the element, in most cases, is the central atom.

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