Question

(a) In the modern Periodic Table, why does cobalt with higher atomic mass of 58.93 appear before nickel having lower atomic mass of 58.71 ?
(b) Why could no fixed position be given to hydrogen in Mendeleev’s periodic table ?

Lakhmir Singh Chemistry Class 10

Answer 1

(a) Cobalt comes first because as we know the modern periodic table is based on the increasing order of atomic number. So the atomic number of cobalt is 27 and nickel is 28. That’s why cobalt comes first in the modern periodic table.

(b) The Electronic configuration of Hydrogen is similar to that of alkali metals. Just like alkali metals, it reacts with halogens, Oxygen and Sulphur to form compounds of similar formulae. On the other hand, Hydrogen reacts with metals and non-metals just like halogens. Due to this anomalous behaviour of Hydrogen, Mendeleev was unable to fix a position for it...

Answer 2

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