The element heavier than cobalt but lighter than copper is Nickel (Ni).

Here's why:

* Atomic Number: Elements are arranged on the periodic table by their atomic number, which represents the number of protons in their nucleus.

* Cobalt (Co): Atomic number 27

* Nickel (Ni): Atomic number 28

* Copper (Cu): Atomic number 29

Therefore, nickel fits perfectly between cobalt and copper in terms of weight.