Question

What do you understand by the term "non-stoichiometric hydrides"? Do you expect this type of the hydrides to be formed by alkali metals? Justify your answer.

Short answer

Non-stoichiometric hydrides have variable hydrogen ratios and are formed by metals like transition metals, not alkali metals. Alkali metals form stoichiometric hydrides instead.

Step-by-step solution

Understand the Definition

Non-stoichiometric hydrides are compounds in which the hydrogen ratio to the other element is not a simple integer ratio. These hydrides often have variable compositions and possess metal-like properties.

Analyze the Formation of Non-Stoichiometric Hydrides

Non-stoichiometric hydrides form when there is a large difference in the hydrogen to metal ratio, typically in transition metals, rare earth, or actinide metals. These hydrides allow for some flexibility where the number of hydrogen atoms can vary without altering the metal framework significantly.

Examine Alkali Metals and Their Hydride Formation

Alkali metals which are Group 1 elements, such as lithium, sodium, and potassium, form stoichiometric hydrides (e.g., LiH, NaH) with a fixed ratio of one hydrogen atom per metal atom. Alkali metals' simple crystal structures do not typically sustain non-stoichiometric hydride formation.

Conclude with a Justification

Since non-stoichiometric hydrides are typically associated with transition metals and require a flexible metal framework, alkali metals do not form such hydrides. Their crystal structures and bonding do not support variable hydrogen ratios.

Key concepts

Hydride Formation

Hydride formation occurs when hydrogen combines with another element, creating a compound called a hydride. This process depends on the type of metal and its ability to bond with hydrogen.

• Stoichiometric Hydrides: These hydrides have a fixed, integral ratio of hydrogen to the other element. An example is lithium hydride (LiH), where one lithium atom bonds with one hydrogen atom.
• Non-Stoichiometric Hydrides: These have a variable ratio of hydrogen to the metal, not following a strict integer ratio. This is common with transition metals, where the structure allows for diverse hydrogen inclusion.

Hydride formation is determined by the type of metal and its electron configuration, defining how it interacts and holds hydrogen atoms.

Alkali Metals

Alkali metals belong to Group 1 on the periodic table and include elements such as lithium, sodium, and potassium. These metals are characterized by their high reactivity and tendency to form ionic bonds.

• Hydride Formation: Alkali metals typically form stoichiometric hydrides, where one hydrogen atom bonds with one metal atom, resulting in compounds like NaH.
• Crystal Structure: Alkali metals have simple crystal structures that do not support variability in hydrogen numbers, which means they cannot form non-stoichiometric hydrides.

Their nature leads to predictable chemical formulas, unlike the variable hydrides of some other elements.

Transition Metals

Transition metals are known for their ability to form a wide range of compounds due to the versatility of their d-orbitals in bonding. This group includes metals like iron, manganese, and nickel, pivotal in forming non-stoichiometric hydrides.

• Hydride Flexibility: Transition metals can accommodate varying amounts of hydrogen without changing the underlying metal structure.
• Metallic Properties: The hydrides of transition metals often retain conductivity and other metallic properties, distinguishing them from simple ionic hydrides.

This flexibility allows transition metals to form complex and variable compositions, which are characteristic of non-stoichiometric hydrides.

Crystal Structure

The crystal structure of a metal determines how it can incorporate hydrogen atoms in the formation of hydrides. The arrangement of atoms affects both the chemical properties and the type of hydride formed.

• Simple Structures: Metals with simple crystal structures, like alkali metals, tend to form stoichiometric hydrides.
• Complex Structures: Transition metals have complex lattice structures that allow for non-stoichiometric hydrides, accommodating variable hydrogen ratios.

Understanding the crystal structure is crucial for predicting the types of hydrides a metal can form and their properties.

Chemical Bonding

Chemical bonding describes the interactions that hold atoms together in a compound, and it plays a vital role in the formation of hydrides.

• Ionic Bonds: Alkali metal hydrides typically involve ionic bonding, where electrons are transferred to hydrogen, making stoichiometric hydrides.
• Metallic Bonds: In transition metals, metallic bonding allows electrons to move freely, facilitating the formation of non-stoichiometric hydrides.

The type of bonding influences not only the formation process but also the physical and chemical properties of the resulting hydrides.