why is cobalt based before nickel even though it has more mass and according to mendeleev it should be placed in the increasing size bof atomic weights.there is a video explanationj given but it is completely confused it is as if they didnt how to explain it this question should be answered only by certified meritnation experts only

Placing of cobalt before nickel is one of the demerits of Mendeleev’s periodic table. In some cases Mendeleev placed elements according to similar chemical properties and not in increasing order of their atomic masses. Thus, the position of these elements was not justified e.g. cobalt (58.9) was placed before nickel (58.7), tellurium(128) was placed before iodine(127).