Assertion (A): Cobalt was placed before nickel in the Mendeleev periodic table.
Reason (B): Cobalt has less atomic mass than nickel.

The correct answer is: A is true but R is false.

There are instances in Mendeleev's Periodic Table when an element with a higher atomic mass is placed before an element with a lower atomic mass. This was done to ensure that elements with similar chemical properties were put in the same group. Due to this, cobalt (58.93) was placed before nickel (58.69). Hence, the assertion is true. But the reason is false.

Therefore, the assertion is true but the reason is false.