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Lewis Structures: Expanded Octet PCl5

November 30, 2012

by Janet Gray Coonce MS


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Now let’s draw the Lewis dot structure for PCl5, phosphorous pentachloride.


Lewis Phosphorous pentachloride

The first step is to draw phosphorous (P)Pin the center.  Phosphorus is in group VA in the periodic table and has 5 valence electrons.  Chlorine (Cl) is a halogen.  The halogens are in periodic table group VIIB and have 7 valence electrons.  The 5 chlorine (Cl) atoms are on the right in the periodic table and are therefore more electronegative.  They will be distributed evenly around the central phosphorus (P) atom.  The next step is to distribute the valence electrons around each of the atoms.  I recommend doing this on the Lewis dot drawing by drawing dots to represent the valence electrons of each atom.


In this drawing, I have drawn 7 valence electrons around each chlorine atom and 5 valence electrons around the phosphorus.  The lone electron of each chlorine atom aligns with the lone electrons on the phosphorus atom.



In this step a line is drawn to connect the dots representing the sharing of the lone electrons in a bond between the P and Cl atoms.



This is the same drawing but it was cleaned up to make a neater presentation for the Lewis dot structure for PCl5.  Now if we count electrons around each atom, we see that each of the chlorine atoms has a complete octet of 8 valence electrons.  This means that the chlorine atoms are stable and are therefore “happy.” Smile

But look at the phosphorus atom there are 2 x 5 = 10 valence electrons.  This is referred to as an expanded octet.  Atoms in period 3 and below in the periodic table may expand their octet because they have d orbitals.  This is in contrast elements with atomic numbers of 1 – 10.  That includes the elements in period 1 (H and He) and elements in period 2 (Li, Be, B, C, N, O, F and Ne).  The first 10 elements of the periodic table do not have d orbitals and cannot expand their valence electrons outside of their duet (H and He) or octet for those in period 2.



Transcribed by James C. Gray, MD FACOG

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