Question: Which scientist theorized that the eighth period of the period table of elements would conform with prior periods?
About the article "Periodic table", section "Further periodic table extensions".

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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.
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.
Seven periods of elements occur naturally on Earth.
For period 8, which includes elements which may be synthesized after 2012, see the extended periodic table.
A group in chemistry means a family of objects with similarities like different families.
There are 7 periods, going horizontally across the periodic table. 
Period 2 is the first period in the periodic table that periodic trends can be drawn from.
Period 1, which only contains two elements (hydrogen and helium) is too small to draw any conclusive trends from it, especially because the two elements behave nothing like other s-block elements.
Period 2 has much more conclusive trends.
For all elements in period 2, as the atomic number increases, the atomic radius of the elements decreases, the electronegativity increases, and the ionization energy increases.
Period 2 only has two metals (lithium and beryllium), making it the least metallic period (period 1 has no metals, but as mentioned above, only has two elements, so it would be impossible to have more metals) and the most nonmetals, with four.
The elements in period 2 often have the most extreme properties in their respective groups; for example, fluorine is the most reactive halogen, neon is the most inert noble gas, and lithium is the least reactive alkali metal.
All period 2 elements completely obey the Madelung rule; in period 2s, lithium and beryllium fill the 2s subshell, and boron, carbon, nitrogen, oxygen, fluorine, and neon fill the 2p subshell.
The period shares this trait with periods 1 and 3, none of which contain transition elements or inner transition elements, which often vary from the rule. 
A period 8 element is any one of 46 hypothetical chemical elements (ununennium through unhexquadium) belonging to an eighth period of the periodic table of the elements.
Sometimes, elements 169 to 172 are also considered to be in period 8 as their outermost electrons fill the 8p3/2 subshells, despite behaving chemically more like the period 9 elements.
They may be referred to using IUPAC systematic element names.
None of these elements have been synthesized, and it is possible that none have isotopes with stable enough nuclei to receive significant attention in the near future.
It is also probable that, due to drip instabilities, only the lower period 8 elements are physically possible and the periodic table may end soon after the island of stability at unbihexium with atomic number 126.
The names given to these unattested elements are all IUPAC systematic names.
If it were possible to produce sufficient quantities of sufficiently long-lived isotopes of these elements that would allow the study of their chemistry, these elements may well behave very differently from those of previous periods.
This is because their electronic configurations may be altered by quantum and relativistic effects, as the energy levels of the 5g, 6f, 7d and 8p1/2 orbitals are so close to each other that they may well exchange electrons with each other.
This would result in a large number of elements in the superactinide series that would have extremely similar chemical properties that would be quite unrelated to elements of lower atomic number. 
This is a version of the periodic table of the chemical elements.
For the natural isotopic composition of each element it shows "name", "atomic number", "symbol", "atomic weight" (or relative atomic mass) and a link to the element's "isotopes".
It also has, by keyed markings, a notification of "occurrence", "state of matter" and "metallic character".
It is named extended because it shows elements that are theoretical only (in period 8 an 9), and large means that the table might exceed page width.
The presentation is "wide" which means that there are no table parts separated (as is more common in print).
The current standard table contains 7 periods, culminating in ununoctium, which has atomic number 118.
The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.
If further elements with higher atomic numbers than this are discovered, they will be placed in additional periods, laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements concerned.
Any additional periods are expected to contain a larger number of elements than the seventh period, as they are calculated to have an additional so-called g-block, containing 18 elements with partially filled g-orbitals in each period.
An eight-period table containing this block was suggested by Glenn T. Seaborg in 1969.
It is not clear how many elements are physically possible, if period 8 is complete, or if there is a period 9.