Phosphorus pentachloride (PCI5) is an exception to the octet rule because it has more than 8 electrons around the valence shell. The chlorine atoms (PCI3) obey the octet rule because the atom has five electrons and need three to fulfill its octet, while the phosphorus atom does not. Phosphorus (PCI5) can form an expanded octet containing ten electrons in the 3d orbitals.

is this correct?

I would delete the very last part about 3d orbitals and just end with 10 electrons. Also, this:

1 hour ago, Rachel Maddiee said:

The chlorine atoms (PCI3) obey the octet rule because the atom has five electrons and need three to fulfill its octet, while the phosphorus atom does not.

Doesn't make a lot of sense. The chlorine atoms in PCl₃ and PCl₅ have 7 valence electrons. Where did you get 5 from? Besides this, your answer is mostly okay but you could explain it a little better. For example, why does PCl5 need to have an expanded octet around the phosphorus? This is the crux of the question, but I don't think you've given a good enough explanation for why it is. 

n order for the P atom to form bonds with 5 Cl atoms, it must have an expanded octet. 

22 minutes ago, Rachel Maddiee said:

n order for the P atom to form bonds with 5 Cl atoms, it must have an expanded octet. 

 

Yes, though I wouldn't recommend directly copy pasting from another website when their background is quite so blue if you don't want to be caught out plagiarising (I assume that came from here https://lavelle.chem.ucla.edu/forum/viewtopic.php?t=23108). You should try and think about it yourself next time.  

Yes, I’m aware. I couldn’t find anything in my reference book so I was trying to find a better explanation online. 

Phosphorus pentachloride (PCI5) is an exception to the octet rule because it has more than 8 electrons around the valence shell. Phosphorus (PCI5) can form an expanded octet containing ten electrons. The central phosphorus atom is bonded to five chlorine atoms. While PCI3 contains a phosphorus atom with an octet.
 

How is this?