2 limitation of octet rule,pls

In AL, I think you should remember BF3 and PCl5, these 2 examples. Please refer to the 2 references mentioned. The following text is copied from the 1st reference.

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Limitation of octet rule

The forth-mentioned method of drawing Lewis structure is only limited to the elements in Period 2. This is because, for principal quantum number 2, there are only 2s, 2px, 2py and 2z orbitals available. Each orbital can accommodate 2 electrons and makes up to a total of 8.
Though the largest no. of valence electron would be 8, the no. can be less than 8.
For example, in NO2, O N O
- +
there are totally ( 5 + 6 + 6 ) = 15 valence electrons which is an odd number. It is
not possible for all atoms to have an octet structure. Actually, there are 8 electrons about each O atom and only 7
eletrons about N.
However, for period 3 elements, there are 3d orbitals available which is capable to accommodate 10 more
electrons. This is why P can form both PCl3 and PCl5 but N can only form NCl3. This phenomenon is called
expansion of octet (or expansion of coordination sphere) and will be explained in the chapter of hybridization
theory.
For those which does not obey octet rule, e.g. PCl5, SO2, another method have to be used to draw the Lewis
structure.
It is assumed that all valence electrons are capable to form bond with peripheral atoms. And the peripheral will
form the required number of bond to achieve octet.
e.g. in PCl5, P has 5 valence electrons and each Cl is capable to form 1 single bond. Therefore, the Lewis structure
will become P
Cl
Cl Cl
Cl Cl
.
e.g. in SO2, S has 6 valence electrons and each O is capable to form 1 double bond. Therefore, the Lewis structure
will become O S O.
e.g. in SO4
2- ion, S has 6 valence electrons ; 2 O atoms with 0 formal charge each forming 2 bond ; 2 O atoms with
-1 formal charge each forming 1 bond. Therefore, the Lewis structure will become S
O O
O- O-.
e.g. XeF4 is a noble gas compound not obeying octet rule. Xe has 8 outermost electrons, each F is capable to form
1 bond. As a result, 4 electrons about Xe will remains as the lone pairs. Therefore, the Lewis structure will
become Xe FF
FF
.
However, for some structures do not obey octet rule, sometimes memorization is required.