Mendeleev assigned eka-manganese an atomic mass of 100. He predicted the element in 1871 as a missing chemical relative positioned below manganese in his periodic system.
The name “eka-manganese” used Mendeleev’s Sanskrit-based naming convention: eka means “one,” indicating a position one place below a known element. The prediction was not merely a placeholder; Mendeleev also anticipated chemical properties for the undiscovered element and estimated its atomic mass.
Eka-manganese was later identified with technetium, element 43. Technetium was not isolated until 1937, when Carlo Perrier and Emilio Segrè produced it from molybdenum bombarded with deuterons. Because technetium has no stable isotopes, its modern atomic-mass figures are usually discussed through isotope mass numbers rather than a naturally occurring average atomic weight.
The number 100 therefore belongs to Mendeleev’s nineteenth-century prediction, not to technetium’s modern periodic-table value. That distinction explains why modern isotope numbers such as 97 and 98 should not be substituted for the historical answer.