What are interstitial hydrides?

Hydrides are compounds that include hydrogen atoms combined with other elements. The types of hydrides are defined by the nature of the bond between hydrogen and those other elements, with each type possessing distinctive properties and structural characteristics.

• Molecular (Covalent) Hydrides: These hydrides involve covalent bonding and usually form between hydrogen and non-metals. Water (\(H_2O\) ) and methane (\(CH_4\) ) are classic examples, with hydrogen sharing electrons with other non-metal atoms.
• Saline (Ionic) Hydrides: Formed between hydrogen and highly electropositive metals, such as those from the s-block of the periodic table. Hydrogen acts as a hydride ion (\(H^-\) ) and the compound exhibits ionic bonding. For example, sodium hydride (\(NaH\) ) is an ionic hydride.
• Metallic (Interstitial) Hydrides: These develop when hydrogen atoms occupy interstitial spaces in a metallic lattice. Typical of transition metals, these hydrides do not conform neatly to the rules of ionic or covalent bonding and showcase both metallic and non-stoichiometric attributes.

It is important for students to recognize these distinctions, as each type's properties and reactions are influenced heavily by their bonding and structure.

A metallic lattice is a highly ordered structure of metal atoms arranged in a three-dimensional geometric pattern. Within this structure, metal atoms donate some of their outer electrons to a 'sea' of electrons, which are delocalized and free to move throughout the lattice. This characteristic facilitates various properties, such as electrical conductivity and malleability.

Role of the Metallic Lattice in Hydrides

In the context of interstitial hydrides, the hydrogen atoms are small enough to fit into the spaces between metal atoms in the metallic lattice. These gaps, known as interstices, allow hydrogen to be 'hosted' without significantly disrupting the metallic structure. The result is a material that retains metallic properties while incorporating hydrogen, a process that can enhance characteristics such as strength and reduce the propensity for embrittlement.

Non-stoichiometric compounds do not adhere to the fixed integer ratios normally seen in chemical formulas. For many compounds, stoichiometry dictates a precise proportion of atoms, but non-stoichiometric compounds like interstitial hydrides can have a variable composition.

• Their composition often depends on conditions like temperature and pressure.
• They may exhibit ranges of composition where they retain the same structure even as the relative amount of hydrogen varies.
• This ratio variance is due to the metallic lattice's ability to accommodate varying amounts of hydrogen without changing its overall structure.

Non-stoichiometry is a critical concept for students to understand as it underpins many of the unique properties of materials, such as the ability to absorb hydrogen and other small atoms without altering their basic metallic structure. Common examples include 'defect structures' like metal oxides, where the oxygen to metal ratio can fluctuate.