Problem 25 - Chemistry: The Central Science in SI Units, Global Edition

Question

Identify the following hydrides as ionic, metallic, or molecular: \((\mathbf{a}) \mathrm{H}_{2} \mathrm{~S},(\mathbf{b}) \mathrm{LiH},(\mathbf{c}) \mathrm{VH}_{0.56}\).

Step-by-Step Solution

Answer

H2S is a molecular hydride due to covalent bonding between non-metal elements. LiH is an ionic hydride due to ionic bonding between a metal (lithium) and a non-metal (hydrogen). VH0.56 is a metallic hydride, as it is a transition metal alloy containing vanadium and hydrogen.

Step 1: Examine element properties and electronegativity difference for H2S

First, examine hydrogen sulfide (H2S). Hydrogen has an electronegativity value of 2.20, and sulfur has an electronegativity value of 2.58. The element sulfur is a non-metal that bonds covalently with hydrogen, so the resulting hydride will be molecular.

Step 2: Determine the hydride type for H2S

Taking into account that H2S is formed through covalent bonding, it will be classified as a molecular hydride.

Step 3: Examine element properties and electronegativity difference for LiH

Next, examine lithium hydride (LiH). Lithium has an electronegativity of 0.98 and hydrogen has an electronegativity of 2.20. Lithium is a metal, which usually forms ionic bonds when reacted with non-metals like hydrogen. Their electronegativity difference is relatively large compared to that of H2S, which supports the formation of an ionic bond.

Step 4: Determine the hydride type for LiH

Taking into account that LiH is formed through ionic bonding, it will be classified as an ionic hydride.

Step 5: Examine element properties for VH0.56

Lastly, examine vanadium hydride (VH0.56). Vanadium is a transition metal. To classify the compound as either ionic or metallic, we have to consider that vanadium hydride is a transition metal alloy and not necessarily a true chemical compound. Since hydrogen is a non-metal, it's difficult to consider this as a purely metallic or ionic bond.

Step 6: Determine the hydride type for VH0.56

Taking into account the properties of vanadium and hydrogen, and considering that VH0.56 is a transition metal alloy, it will be classified as a metallic hydride. To summarize, H2S is a molecular hydride, LiH is an ionic hydride, and VH0.56 is a metallic hydride.

Key Concepts

Ionic Bonding

Ionic bonding is a type of chemical bond formed through the electrostatic attraction between atoms with opposite charges. Typically, this occurs between metals and non-metals. Metals tend to lose electrons, thus acquiring a positive charge, while non-metals tend to gain electrons, acquiring a negative charge. This transfer of electrons leads to the formation of ions.

• For example, in lithium hydride (LiH), lithium (Li) is a metal that loses an electron to become a positively charged ion, and hydrogen (H) gains that electron, becoming negatively charged.
• The significant difference in electronegativity between lithium and hydrogen supports this transfer, resulting in an ionic bond.

The strength of ionic bonds contributes to the high melting and boiling points of ionic compounds. Unlike covalent bonds, where electrons are shared, ionic bonds involve a full transfer of electrons.

Molecular Compounds

Molecular compounds are composed of molecules formed by atoms of two or more non-metals. In these compounds, atoms share electron pairs, resulting in covalent bonds. This sharing happens because the atoms involved have similar electronegativities, meaning neither atom strongly attracts or repels the electrons.

• An example is hydrogen sulfide (H2S), where hydrogen and sulfur atoms share electrons to achieve stable electron configurations.
• Molecular compounds usually have lower melting and boiling points compared to ionic compounds because the intermolecular forces holding them together are weaker.

Molecular hydrides, like H2S, exhibit distinct properties such as low solubility in water and electrical insulation characteristics because the electrons are tightly bound to the molecules, restricting their flow.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons when bonded with another atom. The difference in electronegativity between two atoms can predict the type of bond that will form.

• In LiH, the difference in electronegativity between lithium (0.98) and hydrogen (2.20) is significant, promoting the formation of an ionic bond.
• In contrast, the smaller electronegativity difference between hydrogen (2.20) and sulfur (2.58) in H2S leads to a covalent, sharing relationship between the atoms.

Understanding electronegativity helps us anticipate the behavior and properties of materials, such as their solubility and electrical conductivity. Electronegativity values are crucial in predicting whether a compound will be ionic, covalent, or metallic.

Metallic Bonding

Metallic bonding occurs between metal atoms, allowing them to share many free-floating electrons. This bond type results in properties like conductivity, ductility, and malleability. The free electrons, often referred to as "a sea of electrons," are not bound to any specific atom, enabling the metal to conduct electricity and heat effectively.

• Vanadium hydride (VH0.56) exemplifies a metallic bond, despite the presence of hydrogen. The metal atoms in an alloy like this one share electrons amongst themselves.
• This bond allows for a wide range of mechanical properties, typical for transition metals and their alloys.

Metallic bonds also provide structural stability and durability, making metals essential in many applications. The delocalized nature of electrons permits layers of metal atoms to slide over one another, accounting for the metallic luster and resilience.