The Lewis structure of ICl3 is a drawing or model chemists use to predict the geometry of the molecule. ICl3 is one of the compounds that don't follow the octet rule, as the iodine at the center of this model accepts 10 electrons.
Each chlorine atom in the molecule provides seven valence electrons, for a total of 21. The iodine also provides seven valence electrons. Therefore, the Lewis structure must account for 28 total valence electrons.
In drawing Lewis structures, the most electronegative atom, iodine in this case, is at the center. The drawing then includes the three chlorine atoms, spaced at an even distance around the center ion. Each chlorine atom bonds to the iodine ion using two electrons. The octet rule dictates that three more pairs of electrons surround each chlorine ion. This leaves four electrons that the model maker places around the iodine, so 10 electrons surround the ion instead of the normal eight.
When working with elements in periods two and three of the periodic table, Lewis structures work only with the p-orbital. Elements in lower periods have the d-orbital available, allowing them to accept the additional electrons, as occurs with iodine in ICl3. This same phenomenon allows other elements, such as sulfur, to form compounds using one of several different ionization states.