The characteristic that is needed for an atom to have an expanded octet is for the atom to have an empty d-orbitals.
According to the octet the sum of all the shared and unshared valence electrons about an atom must total 8 to have a stable species, which translates to 4 covalent bonds. However, some elements violate the octet rule and have what we call the expanded octet (the atom ends up with more than an octet); examples of such elements are sulfur, silicon, phosphorus and chlorine.
