Question

Identify the following hydrides as ionic, metallic, or molecular: (a) \(\mathrm{B}_{2} \mathrm{H}_{6}\), (b) \(\mathrm{RbH}\), (c) \(\mathrm{Th}_{4} \mathrm{H}_{1.5}\)

Short answer

(a) \(\mathrm{B}_{2} \mathrm{H}_{6}\) is a molecular hydride, (b) \(\mathrm{RbH}\) is an ionic hydride, and (c) \(\mathrm{Th}_{4} \mathrm{H}_{1.5}\) is a metallic hydride.

Step-by-step solution

Determine the type of \(\mathrm{B}_{2} \mathrm{H}_{6}\)

First, let us analyze the given hydride, \(\mathrm{B}_{2} \mathrm{H}_{6}\). Boron (B) is a metalloid and hydrogen (H) is a nonmetal. The compound is formed by the B-H covalent bonds. Since the bond is between two nonmetals, it is a molecular hydride.

Determine the type of \(\mathrm{RbH}\)

Next, let's analyze the given hydride \(\mathrm{RbH}\). Rubidium (Rb) is an alkali metal located in Group 1 in the periodic table, and hydrogen (H) is a nonmetal. The bond between rubidium and hydrogen is an ionic bond due to the difference in electronegativity between the metal and nonmetal. Therefore, \(\mathrm{RbH}\) is an ionic hydride.

Determine the type of \(\mathrm{Th}_{4} \mathrm{H}_{1.5}\)

Lastly, let's analyze the given hydride \(\mathrm{Th}_{4} \mathrm{H}_{1.5}\). Thorium (Th) is an actinide metal, and hydrogen (H) is a nonmetal. In metallic hydrides, metal atoms have valence electrons that are shared with H atoms in a delocalized manner, similar to what happens in a metallic bond. Since thorium is an actinide metal and forms a hydride with hydrogen, \(\mathrm{Th}_{4} \mathrm{H}_{1.5}\) is a metallic hydride. In conclusion, the given hydrides can be classified as follows: (a) \(\mathrm{B}_{2} \mathrm{H}_{6}\) is a molecular hydride. (b) \(\mathrm{RbH}\) is an ionic hydride. (c) \(\mathrm{Th}_{4} \mathrm{H}_{1.5}\) is a metallic hydride.

Key concepts

Ionic Hydrides

Ionic hydrides are formed when hydrogen combines with strong alkaline earth metals and alkali metals like lithium, sodium, potassium, and rubidium, as seen in the compound \( \mathrm{RbH} \). In these hydrides, hydrogen takes on the form of the hydride ion \( \mathrm{H}^- \), completing an ionic bond. This bond results from the significant difference in electronegativity between the metal and hydrogen. Here, electrons are essentially transferred from the metal to the hydrogen.
Typically, ionic hydrides possess a high melting point and crystallize in a much more structured fashion compared to other hydride types. They are powerful reducing agents, making them extremely reactive with water to produce hydrogen gas and the corresponding hydroxide. Ionic hydrides are crucial in various chemical reactions, especially in the synthesis of organometallic compounds, where they act as hydrogen sources or reducing agents.

Metallic Hydrides

Metallic hydrides, such as \( \mathrm{Th}_{4} \mathrm{H}_{1.5} \), consist of hydrogen atoms embedded in metal lattices. These hydrides are most commonly formed with transition and rare earth metals. The hydrogen atoms occupy interstitial sites within the metal lattice structure, leading to interstitial hydrides.
This type of hydride is formed through the bonding nature in wherein hydrogen atoms are delocalized and shared among a "sea" of metallic atoms. This delocalization imparts significant stability and often unique magnetic properties.
Metallic hydrides have several applications in fields such as hydrogen storage, because of their ability to absorb and release large volumes of hydrogen, and in nuclear technologies given their high neutron moderation capability. These properties make them invaluable for technologies aiming for efficient energy solutions.

Molecular Hydrides

Molecular hydrides occur when hydrogen binds with nonmetals, as exemplified by \( \mathrm{B}_{2} \mathrm{H}_{6} \), or with elements of similar electronegativity, resulting in covalent bonds. These are generally volatile compounds due to the relatively weak intermolecular forces present.
Molecular hydrides usually exist primarily as gases or volatile liquids at room temperature. The covalent bonds in these hydrides create donor-acceptor interactions, where hydrogen either donates or accepts a pair of electrons. One striking feature of molecular hydrides is their diversity in structure and reactivity, which is largely dependent on the electron arrangements of the participating elements.
They play a vital role in organic chemistry and industrial processes, commonly acting as reducing agents, as in the case of diborane \( \mathrm{B}_{2} \mathrm{H}_{6} \), or being utilized in energy production systems such as fuel cells. The flexible chemical behavior of molecular hydrides makes them suitable for a wide range of practical applications.