The Periodic table orders elements based on their atomic number not by their atomic mass even though this order usually coincides with the atomic masses of the elements. Occasionally however, this is not the case as is proven by Cobalt and Nickel.
The atomic number of an element refers to the number of protons that the element has in its nucleus. Cobalt has 27 protons whilst Nickel has one more proton at 28 which is why it comes after Cobalt.
