Final answer:
Among the given choices, Mg2+ has the highest charge density due to its +2 charge and its small ionic radius, resulting in a high charge density for the species. The correct option is option 4
Step-by-step explanation:
The species with the greatest charge density is the one with the smallest radius and the highest charge. Charge density increases as the charge of the ion increases and as the size of the ion decreases. Therefore, we need to consider the charge and size of the ions in each species provided: O2-, Cr, K+, Ca2+, and Mg2+.
Considering the ions, O2- has a -2 charge and is relatively small, resulting in a high charge density. K+ has a +1 charge and is larger, so it has a lower charge density. For Ca2+ and Mg2+, both have a +2 charge, but Mg2+ is smaller, giving it a higher charge density compared to Ca2+. Chromium (Cr) typically exists in multiple oxidation states, but without a specific charge stated, it can't be directly compared.
Therefore, considering their sizes and charges, Mg2+ has the highest charge density among the given choices due to its +2 charge and small ionic radius.