Question

Identify the following hydrides as ionic, metallic, or molecular: (a) \(\mathrm{BaH}_{2}\), (b) \(\mathrm{H}_{2} \mathrm{Te}\), (c) TiH \(_{1,7}\).

Short answer

The hydrides are classified as follows: (a) Barium hydride (BaH2) is an ionic hydride, (b) Hydrogen telluride (H2Te) is a molecular hydride, and (c) Titanium hydride (TiH1.7) is a metallic hydride.

Step-by-step solution

Find electronegativity difference

Determine the electronegativity values for each element in the compound and calculate the difference between them. This will indicate if the bond is ionic or covalent. Electronegativity differences greater than about 1.7 indicate ionic compounds, while smaller differences are suggestive of covalent bonds. (a) Barium hydride (BaH2) Electronegativity of Ba: 0.89 Electronegativity of H: 2.20 Difference: 2.20 - 0.89 = 1.31 (b) Hydrogen telluride (H2Te) Electronegativity of H: 2.20 Electronegativity of Te: 2.1 Difference: 2.20 - 2.1= 0.1 (c) Titanium hydride (TiH1.7) Electronegativity of Ti: 1.54 Electronegativity of H: 2.20 Difference: 2.20 - 1.54 = 0.66

Identify ionic, metallic or molecular hydrides

Based on the electronegativity differences and the elements' position in the periodic table, we can classify the hydrides as follows: (a) Barium hydride (BaH2) - The electronegativity difference is 1.31, suggesting a polar covalent bond. However, considering that barium is an alkali earth metal and it easily forms ionic bonds with more electronegative elements such as hydrogen, we can classify BaH2 as an ionic hydride. (b) Hydrogen telluride (H2Te) - The electronegativity difference is very small (0.1), indicating a covalent bond. Tellurium is in the same group as oxygen and sulfur and forms hydrides similar in structure to water and hydrogen sulfide. Therefore, H2Te is a molecular hydride. (c) Titanium hydride (TiH1.7) - The electronegativity difference is 0.66, indicating a covalent bond. Titanium is a transition metal and can form metallic hydrides that have both covalent and metallic characteristics. Thus, TiH1.7 is a metallic hydride.

Key concepts

Ionic Hydride

Ionic hydrides are compounds in which hydrogen is bonded with a more electropositive metal, typically from the alkali or alkaline earth metals. In these compounds, like barium hydride (\textbf{BaH}\(_2\)), the large difference in electronegativity between the metal and hydrogen leads to the transfer of electrons from metal to hydrogen. This creates a positively charged metal ion (cation) and a negatively charged hydride ion (anion).

Despite the calculated electronegativity difference being less than the typical threshold for ionic compounds, the metallic character of the metal and hydrogen's position as a non-metal, along with its tendency to gain electrons and form a hydride ion, allow barium hydride to exhibit ionic bonding characteristics. Ionic hydrides are often used as reducing agents in chemical reactions due to their ability to supply hydride ions readily.

Metallic Hydride

Metallic hydrides are formed when hydrogen atoms are inserted into the lattice of metallic elements, particularly transition metals like titanium. The compound titanium hydride (\textbf{TiH}\(_{1.7}\)) is an example of a metallic hydride. The bonding in these hydrides is more complex; it has characteristics of both covalent and metallic bonding.

The transition metal has a lower electronegativity than hydrogen, but not so low as to form an ionic bond. Instead, hydrogen atoms essentially enter the gaps or 'interstices' in the metal lattice, resulting in a more delocalized type of bonding. This gives metallic hydrides unique properties, such as the ability to absorb a large volume of hydrogen, which makes them useful for hydrogen storage and other applications.

Molecular Hydride

Molecular hydrides consist of molecules in which hydrogen is covalently bonded to a non-metal. An example given in the exercise is hydrogen telluride (\textbf{H}\(_{2}\)\textbf{Te}). Here, the small difference in electronegativity between hydrogen and tellurium indicates a non-polar covalent bond. However, since hydrogen and tellurium are not identical, a slight polarity arises leading to a polar covalent bond.

Molecular hydrides often resemble the properties of water because they usually involve group 16 elements (oxygen, sulfur, selenium, tellurium) bonded with hydrogen. Due to the lower electronegativity of tellurium compared to oxygen, hydrogen telluride is less polar than water and is a gas at room temperature. Molecular hydrides are typically volatile compounds and may be used as reducing agents or reactants in organic synthesis.

Polar Covalent Bond

A polar covalent bond occurs when two atoms with a difference in electronegativity share electrons unequally. One atom attracts the shared pair of electrons more strongly, becoming slightly negative, while the other atom becomes slightly positive. This separation of charge creates a dipole moment.

In the context of hydrides, while some ionic character might be present due to a significant difference in electronegativity (as in the case with \textbf{BaH}\(_2\)), the bond is often considered polar covalent if the difference does not exceed the commonly accepted ionic threshold (about 1.7). Polar covalent bonds are essential in chemistry because they determine molecular polarity, influencing the physical and chemical properties of substances, including solubility, melting and boiling points, and reactivity.