1) "The concept of the Expanded Octet occurs in any system that has an atom with more than four electron pairs attached to it. Most commonly, atoms will expand their octets to contain a total of five or six electron pairs, in total. In theory, it is possible to expand beyond those number. The large amounts of negative charge concentrated in small volumes of space prevent those larger expanded octets from forming. When an atom expands its octet, it does so by making use of empty d orbitals that are available in the valence level of the atom doing the expanding. Atoms that do not have empty valence level d orbitals will not be able to expand their octets. The atom that expands its octet in a structure will usually be located in the center of the structure and the system will not use any multiple bonds in attaching atoms to the central atom. The process of expanding octets is strictly a last resort on the part of atoms." Source and further information: http://www.bcpl.net/~kdrews/molegeo/molegeo.html 2) "Expanded octet” refers to the Lewis structures where the central atom ends up with more than an octet, such as in PCl5 or XeF4. In drawing the Lewis structure for PCl5 , there is a total of 40 valence electrons to put in (5 + 5x7 = 40). One can easily see that if the central atom, P, is to be joined to five Cl atoms, P would have 10 electrons instead of the octet." Source and further information: http://student.ccbcmd.edu/~cyau1/121TutorialExpandedOctetSp2006.pdf 3) "The LE model provides a simple and very successful method for modeling covalent compounds based on the arrangement of atoms and valence electrons, both bonding and non-bonding, resulting in an octet for every atom; However, it has some exceptions 1. Molecules with an odd number of electrons 2. Molecules in which an atom has less than an octet 3. Molecules in which an atom has more than an octet." "Molecules containing elements of the 3rd period and beyond may possibly form compounds or ions in which there are more than eight electrons in the valence shell. (i.e. SF6, PCl5)" http://www.vigoschools.org/~mmc3/ap%20lecture/Unit%206%20-%20Chemical%20Bonding/L4%20-%20Exceptions%20to%20the%20Octet%20Rule%20Ch%208.pdf

An octet is set of eight. Expanding the octet means the compound will have more than 8 valence electrons and thus will have more than 8 electrons when the Lewis dot structure for it is drawn. An example of this is Phosphorous PentaChloride(PCl5)