Octet Rule Exception [ENDORSED]

[Luke_Lucido_3B]

Conceptually why does the third row of elements not have to follow octet rule?

[Minu Reddy]

The reason is because the third row in the periodic table corresponds to the third energy level which is n=3. Given that n=3, you can have l values of 0,1,2.
l=2 corresponds to a d sub level and because there are 5 orbitals in a d sub level, you can have a total of 10 electrons which is why elements in the third row of the periodic table are considered to be an exception to the octet rule.

[Chem_Mod]

This above answer is hitting on the right topic but I think I'll expand on it some. The third row elements have valence orbitals consisting of one 3s orbital, and three 3p orbitals, these orbitals are much closer in energy to the first set of d orbitals, the 3d orbitals (though the row 3 elements do not normally occupy the 3d orbitals, they can be accessed during bonding). Because they are similar in energy, in certain circumstances the d orbitals are able to be accessed and enlisted to participate in additional bonding beyond the normal (2 electrons in s, + (2 x 3) electrons in p = 8 electrons for an octet and then 2 electrons or 4 electrons in d = 10 or 12 electrons total for expanded octet). So the 10 electrons that may reside in 5 d orbitals are not the same 10 electrons that may participate in expanded octet bonding.

Just to clarify that point because I think that its an important one that is potentially easy to confuse about expanded octets. The fact that sometimes 10 electrons may participate in bonding in certain row 3 elements is coincidental to the fact that a full 3d shell also holds 10 electrons
Hope that helps!

[Luke_Lucido_3B]

Thank you that helped