Which periodic trend is partially responsible for the observation that the maximum oxidation state of the transition-metal elements peaks near groups 7 and \(8 ?(\mathbf{a})\) The number of valence electrons reaches a maximum at group 8. (b) The effective nuclear charge increases on moving left across each period. (c) The radii of the transition-metal elements reach a minimum for group \(8,\) and as the size of the atoms decreases it becomes easier to remove electrons.

For the transition-metal elements, the number of valence electrons increases from left to right across a period. However, the maximum oxidation state does not necessarily correlate directly with the number of valence electrons. Additionally, the maximum valence electrons for transition-metal elements are not reached in groups 7 and 8, so this statement does not explain the observed trend. #Step 2: Analyzing Option (b)#

The effective nuclear charge is the net positive charge experienced by an electron in the outermost shell of an atom. While it is true that the effective nuclear charge increases from left to right across a period, this statement says it increases moving left, which is incorrect. Therefore, this option does not explain the observed trend in the maximum oxidation state. #Step 3: Analyzing Option (c)#

As we move from left to right across a period, atomic radii generally decrease due to increases in the effective nuclear charge. Smaller atomic radii mean that outer electrons are more tightly bound to the nucleus, making it easier to remove them and form higher oxidation states. This statement aligns with the observed trend that the maximum oxidation states peak near groups 7 and 8. Thus, it is the most likely explanation for the observed trend. #Conclusion# The correct answer to the exercise is: (c) The radii of the transition-metal elements reach a minimum for group 8, and as the size of the atoms decreases, it becomes easier to remove electrons. This periodic trend is partially responsible for the observation that the maximum oxidation state of the transition-metal elements peaks near groups 7 and 8.