Answer
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Work Step by Step
There are a few pairs of elements that are out of order in terms of increasing atomic mass, including:
Cobalt (Co) and Nickel (Ni)
Tellurium (Te) and Iodine (I)
Argon (Ar) and Potassium (K)
The reverse order is necessary because atomic mass is not always a perfect indicator of an element's position on the periodic table. Atomic mass is the sum of the number of protons and neutrons in an atom, but the number of neutrons can vary between atoms of the same element, leading to different isotopes with slightly different atomic masses.
