Metallic hydrides are compounds formed when diatomic hydrogen reacts with transition metals. A notable example is tantalum hydride, which illustrates an important concept in the study of these compounds: the stoichiometric ratio of metals to hydrogen atoms is not always a whole number. This is due to the unique way hydrogen occupies the spaces within the metal lattice structure.

In a metallic hydride, the transition metal atoms, represented by larger spheres, create a lattice framework. The much smaller hydrogen atoms fit into the gaps between these metal atoms. This arrangement can lead to non-integer ratios, such as 0.9, indicating that hydrogen does not always fill the lattice in a simple, whole-number manner. Understanding this behavior is crucial for grasping the properties and formation of metallic hydrides, as it highlights the complexity of their structure and composition.