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Achievements of Mendleeves periodic table
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Explanation:
Achievements of Mendeléev’s periodic table
element with a slightly greater atomic mass were placed
before an element with a slightly lower atomic mass with an inverted sequence so as to group the elements with similar properties. For example, cobalt with atomic mass 58.9 was placed before nickel with atomic mass 58.7.
He left some gaps in the Periodic Table with the prediction of
existence of some elements that were not discovered at that time.
He named the future elements by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group.
For instance, scandium, gallium and germanium, discovered later, have properties
similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively.
Noble gases like helium (He), neon (Ne) and argon (Ar) were discovered
later as they are present in exceptionally low concentrations in the atmosphere due to their inertness.