They do so on the basis that the group 3 elements do not form any ions having a partially occupied d shell and do not therefore exhibit properties characteristic of transition metal chemistry. In this case, only groups 4–11 are regarded as transition metals. This categorisation is however not one of the alternatives considered by IUPAC. Though the group 3 elements show few of the characteristic chemical properties of the transition metals, the same is true of the heavy members of groups 4 and 5, which also are mostly restricted to the group oxidation state in their chemistry. Moreover, the group 3 elements show characteristic physical properties of transition metals (on account of the presence in each atom of a single d electron). Although all elements up to oganesson have been discovered, of the elements above hassium (element 108), only copernicium (element 112), nihonium (element 113), and flerovium (element 114) have known chemical properties, and conclusive categorisation at present has not been reached. Some of these may behave differently from what would be predicted by extrapolation, due to relativistic effects; for example, copernicium and flerovium have been predicted to possibly exhibit some noble-gas-like properties, even though neither is placed in group 18 with the other noble gases. The current experimental evidence still leaves open the question of whether copernicium and flerovium behave more like metals or noble gases. At the same time, oganesson (element 118) is expected to be a solid semiconductor at standard conditions, despite being in group 18. Currently, the periodic table has seven complete rows, with all spaces filled in with discovered elements. Future elements would have to begin an eighth row. Nevertheless, it is unclear whether new eighth-row elements will continue the pattern of the current periodic table, or require further adaptations or adjustments. Seaborg expected the eighth period to follow the previously established pattern exactly, so that it would include a two-element s-block for elements 119 and 120, a new g-block for the next 18 elements, and 30 additional elements continuing the current f-, d-, and p-blocks, culminating in element 168, the next noble gas. More recently, physicists such as Pekka Pyykkö have theorized that these additional elements do not exactly follow the Madelung rule, which predicts how electron shells are filled and thus affects the appearance of the present periodic table. There are currently several competing theoretical models for the placement of the elements of atomic number less than or equal to 172.