Question

why is cobalt placed before nickel on the periodic table of elements even though it has a higher average atomic mass than nickel

Answer 1

Because it has a smaller atomic number. Elements are not lined up according to the higher average atomic mass, but rather according to the higher atomic number.

Answer 2

Just because it has a smaller atomic number. Elements are not lined up according to the higher average atomic mass, but rather according to the higher atomic number.