Answer
There are 3 such instances:
- From $Ar$ to $K$
- From $Co$ to $Ni$
- From $Te$ to $I$
Work Step by Step
Normally, as we move forward in the periodic table, the atomic numbers increase, and so do the atomic weights of the elements.
The atomic weights of the elements are considered out of order relative to the atomic numbers when, as atomic numbers increase, the atomic weights of the elements decrease.
The first instance happens from $Ar (Z=18)$ with atomic weight of about 40 to $K (Z=19)$ with atomic weight of about 39.
The second instance happens from $Co (Z=27)$ with atomic weight of about 58.93 to $Ni (Z=28)$ with atomic weight of about 58.69.
The third instance happens from $Te (Z=52)$ with atomic weight of about 127.60 to $I (Z=53)$ with atomic weight of about 126.90.
