![]() Most of the elements in this column are slightly out of order. Technetium (Tc), the element which belongs between ruthenium and molybdenum (Mo) has no stable isotopes and was not synthesized until 1937. It belongs between ruthenium (Ru) and palladium (Pd). This original prediction was actually borne out in 1923 with the discovery of hafnium. His later tables erroneously placed lanthanum in this spot. Mendeleev soon retreated from this prediction of a heavier analogue of titanium and zirconium. ![]() This table contains several implicit predictions of unknown elements. Mendeleev would use the latter sort of layout before long. Note also the cosmetic difference between Mendeleev's layout and that of modern tables, which order the elements in horizontal rows such that families of elements appear in vertical columns. #Iron periodic table seriesNotes The principle of periodicity is apparent in this first sentence: repetition of chemical properties in a series of elements arranged by atomic weight. Thus Bo (?) appears as an analogue of Bo and Al, as is well known to have been long established experimentally.
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