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How does palladium differ from other platinum-group metals in chemical reactivity?

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rhodium — platinum group Rhodium foil and wire.

Palladium is distinguished from other platinum-group metals by its higher level of chemical reactivity. While the platinum group is generally characterized by a paradoxical resistance to most chemical reagents, palladium is more susceptible to attack by various substances.12

Specifically, palladium is attacked by acids more readily than any other metal in the group. It dissolves slowly in nitric acid and sulfuric acid, and its sponge form can even dissolve in hydrochloric acid when chlorine or oxygen is present. Furthermore, palladium is rapidly attacked by fused alkali oxides and peroxides, as well as by fluorine and chlorine at elevated temperatures.1

Additionally, palladium exhibits a unique capacity to absorb more than 900 times its own volume of hydrogen under suitable conditions, forming a metallic hydride. Among transition metals, it also possesses one of the strongest tendencies to form bonds with carbon.1