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Atomic number (Z) is the number of protons in the nucleus of an atom, and it determines the element's place in the periodic table. Atomic weight is the weighted average mass of the naturally occurring isotopes of an element. It should be noted that the atomic weight of an element will always be greater than or equal to its atomic number, as it is the mass of protons and neutrons combined.

A stable element is one that does not undergo radioactive decay. In this exercise, we shall consider elements up to lead (Z=82), which is the heaviest stable element. We will have to analyze the atomic weights and atomic numbers of these elements.

In order to find the instances where the atomic weights of the stable elements are in reverse order relative to the atomic numbers, we have to compare and analyze both quantities for these elements. We need to find instances where the element with a higher atomic number has a lower atomic weight than the element with a lower atomic number.

There are two instances where the atomic weights of consecutive elements are in reverse order relative to their atomic numbers: 1. Argon (Ar) \(Z=18\) and Potassium (K) \(Z=19\). 2. Cobalt (Co) \(Z=27\) and Nickel (Ni) \(Z=28\). In both these cases, the atomic weight of the element with a higher atomic number is less than that of the element with a lower atomic number.

The explanation for the reverse order in atomic weights relative to the atomic numbers is due to isotopic abundance. The atomic weight of an element is not an integer number, as it is a weighted average of the naturally occurring isotopes' masses of an element. In both instances mentioned above, the relative abundances of the heavier isotope for the lighter element are higher than for the heavier element. This results in the overall atomic weight being higher for the element with a lower atomic number and lower for the element with a higher atomic number. In conclusion, there are two instances where the atomic weights of stable elements up to lead are in reverse order relative to their atomic numbers, and the reason for this is due to the isotopic abundance of the elements involved.