How do you write a lewis structure of PCl3? What about polyatomic ions and atoms that have expanded octet like KClO4?

How do you write the lewis structure for PCl3?  How do you find the number of valence electrons and how do you know where to place those valence electrons as bonds or lone pairs?

What about for a species with a cation and anion like KClO4?  K is a plus one cation and ClO4 is a minus one anion.  Does the Cl use an expanded octet to form bonds with the oxygen atoms?  What period does Cl need to be in to expand its octet?


Valence electrons are the electrons present in the outermost shell of electronic configuration of any element.P is in 5A group and has 5 valnece electrons and Cl is 7A group so has 7 valnece electrons.

learn it from this blog  :


Here is a short video that goes over valence electrons and Lewis diagrams:


Watch that video before you do anything else.

Find the valence electrons of an element by simply counting the columns but skipping the middle part (transition metals)

So, you should count over and find that P has 5, O has 6, and Cl has 7 valence electrons.


Generally, do not give anything an expanded octet unless you have to.  When do you have to?  When you have more than 4 atoms attached to a central atom (e.g. SF6, BrF5) or when you simply have too many electrons (e.g. XeO2, BrF3).  This is very rare for polyatomic ions and most often occurs when fluorine is involved (e.g. PF6-).  Traditionally, many chemists have written polyatomic ions such as sulfate and perchlorate with expanded octets, but modern calculations have shown that to be incorrect.  If you are interested in this advanced topic, see:


If you watch the video above and follow the steps, you should end up with the following Lewis structures:


PCl3, phosphorus is in the middle with one lone pair and is surrounded by single bonds to the three chlorines, each with 3 lone pairs.
KClO4 is ionic. K+ is normally drawn with zero dots because K just lost its only valence electron (although K+ of course has 8 valence electrons just like argon). Perchlorate has chlorine in the middle with zero lone pairs and is surrounded by single bonds to the four oxygens, each with 3 lone pairs.
Good Luck!