<Tr> <Td> Periodic table with eight rows, extended to element 172 </Td> </Tr> <P> It is unclear whether new elements will continue the pattern of the current periodic table as period 8, 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 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 . In all of these it is element 172, rather than element 168, that emerges as the next noble gas after oganesson, although these must be regarded as speculative as no complete calculations have been done beyond element 122 . </P> <P> The number of possible elements is not known . A very early suggestion made by Elliot Adams in 1911, and based on the arrangement of elements in each horizontal periodic table row, was that elements of atomic weight greater than 256 ± (which would equate to between elements 99 and 100 in modern - day terms) did not exist . A higher--more recent--estimate is that the periodic table may end soon after the island of stability, which is expected to centre around element 126, as the extension of the periodic and nuclides tables is restricted by proton and neutron drip lines . Other predictions of an end to the periodic table include at element 128 by John Emsley, at element 137 by Richard Feynman, and at element 155 by Albert Khazan . </P> <Dl> <Dt> Bohr model </Dt> </Dl>

What is the basis of arranging the elements in the modern periodic table