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

Analyze the elements in hydride (a) \(\mathrm{B}_{2}\mathrm{H}_{6}\)

Boron (B) is a metalloid and hydrogen (H) is a nonmetal. Metalloids generally exhibit properties from both metals and nonmetals. In this case, boron forms a covalent bond with hydrogen.

Classify hydride (a) \(\mathrm{B}_{2} \mathrm{H}_{6}\)

Since the bond between boron and hydrogen is covalent, the hydride \(\mathrm{B}_{2}\mathrm{H}_{6}\) is classified as a molecular hydride.

Analyze the elements in hydride (b) \(\mathrm{RbH}\)

Rubidium (Rb) is an alkali metal and hydrogen (H) is a nonmetal. Alkali metals are known to have a strong tendency to donate an electron to form a positive ion. In the case of rubidium and hydrogen, rubidium will donate an electron to hydrogen to form a ionic bond.

Classify hydride (b) \(\mathrm{RbH}\)

Since the bond between rubidium and hydrogen is ionic, the hydride \(\mathrm{RbH}\) is classified as an ionic hydride.

Analyze the elements in hydride (c) \(\mathrm{Th}_{4}\mathrm{H}_{1.5}\)

Thorium (Th) is an actinide metal and hydrogen (H) is a nonmetal. Actinides are known to have variable valency and can form different types of bonds. In this case, thorium will bond with hydrogen in a way that is typically considered to be metallic bonding.

Classify hydride (c) \(\mathrm{Th}_{4}\mathrm{H}_{1.5}\)

Since the bond between thorium and hydrogen is metallic, the hydride \(\mathrm{Th}_{4}\mathrm{H}_{1.5}\) is classified as a metallic hydride. In conclusion, the 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

Molecular Hydrides

Molecular hydrides are compounds formed when hydrogen bonds covalently with non-metals or metalloids. These bonds occur due to the sharing of electrons between the atoms involved.
An example of a molecular hydride is diborane, \( \mathrm{B}_2\mathrm{H}_6 \) where the boron atoms, being metalloids, share electrons with hydrogen atoms to form a stable molecule. Such hydrides are characterized by discrete, covalently bonded molecules in their most stable form. Molecular hydrides are typically volatile and may display complex bonding arrangements, as seen in the multiple bonds between boron and hydrogen atoms within diborane.

Ionic Hydrides

Ionic hydrides are formed when hydrogen gains an electron from an alkali metal or alkaline earth metal to become a hydride ion, \( \mathrm{H}^- \).
This process creates a strong ionic bond as found in rubidium hydride, \( \mathrm{RbH} \), where rubidium donates an electron to hydrogen. These hydrides are made up of positively charged metal ions and negatively charged hydride ions, making them salts. They are often crystalline solids at room temperature and can react violently with water to release hydrogen gas.

Metallic Hydrides

Metallic hydrides consist of hydrogen that non-covalently bonds with transition metals, actinides, or lanthanides, leading to an 'electron sea' that permits electrical conductivity.
An example is \( \mathrm{Th}_4\mathrm{H}_{1.5} \), a hydride of thorium, an actinide. Here, hydrogen is absorbed into the metal lattice where it donates electrons, allowing them to move freely and enforce metallic properties. Unlike ionic or molecular hydrides, these are not composed of discrete hydrogen anions and are instead integrated into the metal's structure, often leading to properties like superconductivity in certain compositions.

Chemical Bonding in Hydrides

Chemical bonding in hydrides varies greatly depending on the elements involved and the type of hydride formed.
In molecular hydrides, covalent bonds dominant as non-metals share electrons. Ionic hydrides exhibit electrostatic attractions between ions within a crystalline lattice, while metallic hydrides feature metallic bonding with delocalized electrons. Understanding the type of bonding helps predict the hydrides' physical and chemical properties such as phase, reactivity, and electrical conductivity.

Alkali Metals

Alkali metals, which include lithium, sodium, potassium, rubidium, cesium, and francium, have one valence electron they readily lose to form positive ions.
This characteristic makes them highly reactive, especially with hydrogen to form ionic hydrides. They are soft metals with low densities and are found in the first column of the periodic table. Their tendency to lose an electron is the key factor in forming ionic compounds like rubidium hydride, \( \mathrm{RbH} \).

Actinides

Actinides are a series of metallic elements from actinium to lawrencium which contain unfilled 5f electron orbitals.
These elements, including thorium, uranium, and plutonium, possess a diversity of oxidation states, allowing them to form complex metallic hydrides. They are often radioactive and have significant importance in nuclear energy. Thorium hydride, \( \mathrm{Th}_4\mathrm{H}_{1.5} \), as a metallic hydride, showcases the actinides' ability to incorporate hydrogen into their metallic lattice.

Metalloids

Metalloids are elements with properties that are intermediate between metals and nonmetals.
Elements such as boron, silicon, and germanium fall into this category. They can form both covalent and metallic bonds, which allows them to create diverse compounds, including certain hydrides. In the context of hydrides, metalloids typically form molecular hydrides through covalent bonding with hydrogen, such as in the compound diborane, \( \mathrm{B}_2\mathrm{H}_6 \).