Question

How do we explain the fact that cobalt comes before nickel in the periodic table, even though cobalt has a higher atomic mass than nickel?

Answer 1

Elements on the periodic table are ordered according to the number of protons an atom of the element has in its nucleus. This is called the atomic number. Cobalt has 27 protons while Nickel has 28 - thus Cobalt is first on the table. Atomic mass, however, has to do with protons and neutrons.