POCl3. Draw with octet rule, and 2 with expanded octet

Question

Can you please also explain the difference between octet and expanded octet.

Answer 1

Cannot easily draw Lewis dot structures on this platform, but I'll try to write a description so that you can draw them: P has 5 valence electrons; O has 6, each Cl has 7 for a total number of valence electrons of 32.

Place the P in the center and attach 3 Cl atoms and the 1 O atom. That takes 8 electrons. Put 3 lone pairs (6 electrons) around each of the attached atoms (i.e. 6 around each Cl and 6 around the O). That now uses all 32 electrons. The P in the center (and all other atoms) now have 8 electrons and obey the octet rule.

The problem is that now the P has a formal charge of +1 and the O has a formal charge of -1. It would be more stable if neither had a formal charge. So, move 2 of the electrons that are around the O and use them to form a double bond to the P. Now P has 10 electrons and this is what we call an expanded octet (more on this below). The O still has 8 electrons associated with it, and now the formal charge on both P and O is zero.

When an element is below the 3rd low (period), it can have an expanded octet because it has d orbitals available. I'm not sure how advanced you are in chemistry, so I won't go into any more detail, but suffice it to say, that the difference between and octet and an expanded octet is just that the expanded octet has more than 8 electrons.

I hope this helps explain the difference.