Animated Solution for Chemistry - d and f-Block Elements: Which one of the following metals forms interstitial hydride easily ?

Have you ever wondered what happens when you expose a solid block of metal to hydrogen gas? It turns out that many transition metals have a fascinating ability to act like a sponge for hydrogen.

Imagine the atomic structure of a metal. The metal atoms are packed tightly in a crystal lattice, but no matter how tightly they pack, there are always tiny empty spaces left between them. These spaces are called interstitial voids. Because hydrogen atoms are incredibly small, they can easily slip into these voids without disrupting the overall crystal structure of the metal. This phenomenon of a solid metal absorbing a gas is known as occlusion, and the resulting compounds are called interstitial hydrides or metallic hydrides.

While it sounds like any metal could just soak up hydrogen, nature has its own strict rules. If we look at the $d$-block of the periodic table, we notice a very peculiar trend. Metals belonging to Groups 7, 8, and 9 absolutely refuse to form hydrides under normal conditions.

This complete lack of hydride formation in this specific region of the periodic table is famously known as the Hydride Gap.

Let's evaluate the options given in our problem based on this rule: Manganese (Mn) belongs to Group 7. Iron (Fe) belongs to Group 8. Cobalt (Co)* belongs to Group 9.

Because all three of these metals fall squarely inside the Hydride Gap, they will not form interstitial hydrides easily.

The Lone Wolf of Group 6

This leaves us with our final candidate: Chromium (Cr). Chromium is a Group 6 element. Interestingly, while the other elements in Group 6 (Molybdenum and Tungsten) do not form hydrides, Chromium is the unique exception. It readily absorbs dihydrogen to form Chromium Hydride ($\text{CrH}$).

Therefore, among the given choices, Chromium is the only metal that easily forms an interstitial hydride. Remembering the Hydride Gap is a fantastic shortcut for solving these types of inorganic chemistry questions in seconds!