Question

In group VIII of the Periodic Table, why does cobalt with atomic mass 58.93 appear before nickel having atomic mass 58.71 ?

Group	I	II	III	IV	V	VI	VII	VIII
OxideHydride	2	2	2, 33	24	2, 53	32	2,7	4
Periods1	1.008							Transition series
2	6.939
3 h8	22.99	24.31	29.98	28.09	30.974	32.06	35.453
4 First series:Second series	39.10263.54	40.0865.37	44.9669.72	47.9072.59	50.9474.92	50.20Se78.96	54.9479.909	55.85 58.93 58.71
5 First series:Second series	85.47107.87	87.62112.40	88.91114.82	91.22118.69	92.91121.60	95.94127.60	99126.90	H101.07 102.91 106.4
6 First series:Second series	132.90	137.34	138.91	178.49	180.95	183.85		190.2 192.2 195.09

Answer 1

Hint: The Cobalt shows similar properties with the group 8 elements while nickel does not do so. That is why Mendleev gave priority to chemical properties than atomic masses at some places.

Complete step by step answer:
This picture is from Mendleev’s periodic table. His periodic table was based on the increasing number of atomic masses. He said that the chemical and physical properties of the elements recur periodically when they are arranged in increasing order of atomic weights.
So, he arranged the elements in increasing order of atomic masses or atomic weights. Thus, accordingly the cobalt should be placed after the nickel but the mendeleev did not do this because he saw that the properties of cobalt resembled the VIII group. The cobalt showed properties similar to its group members. Not only here but even at some other places Mendleev gave importance to the chemical properties instead of atomic masses. So, cobalt came before nickel in the periodic table.
Further, one can also think that the mass is the sum of the number of protons and neutrons. And if we move in Mendleev’s periodic table with an increasing number of protons. Then cobalt has 27 protons while the nickel in its atom has 28 protons. So, this could also be the reason to place cobalt before nickel.

Note: It must be noted that although Mendleev arranged the elements on the basis of atomic masses he did not completely believe in this. He studied physical and chemical properties of the elements and found that arrangement according to atomic masses lead to periodicity in properties of elements. Thus, whenever the properties were matching he even neglected the atomic mass.