Question

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

Short answer

(a) B2H6: Molecular hydride \newline (b) RbH: Ionic hydride \newline (c) Th4H1.5: Metallic hydride

Step-by-step solution

Identify the elements in each hydride and determine their electronegativity difference values.

For each of the given hydrides, we need to identify the elements involved and determine their electronegativity difference values: (a) B2H6: Boron (B) and Hydrogen (H) Boron's electronegativity: 2.04 Hydrogen's electronegativity: 2.20 Electronegativity difference: abs(2.04 - 2.20) = 0.16 (b) RbH: Rubidium (Rb) and Hydrogen (H) Rubidium's electronegativity: 0.82 Hydrogen's electronegativity: 2.20 Electronegativity difference: abs(0.82 - 2.20) = 1.38 (c) Th4H1.5: Thorium (Th) and Hydrogen (H) Thorium's electronegativity: 1.3 Hydrogen's electronegativity: 2.20 Electronegativity difference: abs(1.3 - 2.20) = 0.9

Classify each hydride based on electronegativity differences and periodic table position.

Now, we will classify each hydride as ionic, metallic, or molecular based on the electronegativity difference values and the position of the elements in the periodic table. (a) B2H6: The electronegativity difference between Boron and Hydrogen is 0.16, which is quite small. Additionally, Boron is a non-metal element from group 13 in the periodic table. Given these properties, B2H6 is a molecular hydride. (b) RbH: The electronegativity difference between Rubidium and Hydrogen is 1.38, which is quite large. Rubidium is an alkali metal from group 1 in the periodic table, forming ionic bonds with nonmetals like Hydrogen. Therefore, RbH is an ionic hydride. (c) Th4H1.5: The electronegativity difference between Thorium and Hydrogen is 0.9, which is intermediate. Thorium is an actinide metal, and such metals usually form metallic hydrides. The H1.5 subscript indicates that there is a non-stoichiometric ratio between Thorium and Hydrogen atoms, which is characteristic of metallic hydrides. Hence, Th4H1.5 is a metallic hydride. So, to sum up the classification: (a) B2H6: Molecular hydride (b) RbH: Ionic hydride (c) Th4H1.5: Metallic hydride

Key concepts

Electronegativity Difference

Electronegativity difference plays a crucial role in determining the type of bond formed between atoms in a compound. It is defined as the absolute value of the difference in electronegativity values between two bonded atoms. Electronegativity itself is a measure of an atom's tendency to attract shared electrons in a chemical bond.

When considering hydrides, which are compounds that include hydrogen, the electronegativity difference between hydrogen and the other element can indicate the bond type. If this difference is small (usually less than 0.5), the bond is typically covalent and the compound is classed as a molecular hydride. A large difference (greater than 1.7) implies an ionic bond, thus the compound is an ionic hydride. Values that fall between these ranges can lead to polar covalent or, in the case of some metals, metallic bonds.

Ionic Hydride

Ionic hydrides form when hydrogen bonds with a significantly more electropositive metal, typically an alkali or alkaline earth metal. Since these metals have a much lower electronegativity compared to hydrogen, the difference in electronegativity is large, leading to the transfer of an electron from the metal to the hydrogen.

In an ionic hydride, hydrogen exists as a hydride ion (H⁻), carrying a negative charge. Compounds like Rubidium hydride (RbH) demonstrate ionic characteristics due to the large electronegativity difference, and hence RbH is classified as an ionic hydride. These hydrides are often used as reducing agents in chemical syntheses and have high ionic conductivity.

Metallic Hydride

Metallic hydrides are formed with transition metals, lanthanides, and actinides, wherein hydrogen occupies interstitial sites within the metal lattice. The nature of the bonding in metallic hydrides is complex; it is neither purely ionic nor purely covalent, but it exhibits metallic bonding characteristics with a variable composition.

For example, Thorium hydride (Th4H1.5) has a non-stoichiometric Hydride-Hydrogen ratio, indicative of its metallic hydride nature. Such hydrides can absorb and release hydrogen, making them useful for hydrogen storage and thus important in energy applications, especially those focused on clean energy technology.

Molecular Hydride

Molecular hydrides are covalently bonded compounds where hydrogen is bonded to a non-metal. The electronegativity difference between the hydrogen and the other element is relatively small, resulting in shared electron pairs. Typically, these types of hydrides have discrete molecular structures.

Diborane (B2H6) is an example of a molecular hydride with covalent bonds between boron and hydrogen. The small electronegativity difference allows for a stable molecular structure. Molecular hydrides are prevalent in various chemical reactions and can serve as reagents, fuels, and in the case of diborane, as a rocket fuel due to its high energy content.