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As we learned the wave function numbers and their meaning we learned that we are dealing with a d orbital when l=2. For l to equal 2 the value of n must be at least 3. This means elements in the 3rd period possess a d sub-shell. However, we also learned that the electron configuration for an element like phosphorus is [Ne] 3s2 3p3. There are no electrons within the d subshell. From these observations we can conclude that any element in period 3 or higher possesses an empty d subshell, allowing them to possess more than just the normal octet of electrons.
Any element period 3 and below can have an expanded octet because they can also have d orbitals (10 valence electrons). For example, Phosphorus can have an expanded octet because it is in period 3, while Nitrogen cannot have an expanded octet because it is in period 2.
Expanded octet is when an atom can hold more than 8 valence electrons in a lewis structure. So some atoms like sulfur, phosphorous, or chlorine are more prone to expanded octets. This is something you should simply keep in mind.
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