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Hydrogen (Hydrides)

Hydrides

Hydrides: Hydrides are compounds formed between hydrogen and other elements. Hydrogen combines with almost all elements in the periodic table, except for noble gases. The nature of these compounds depends on the electronegativity difference between hydrogen and the other element.

Classification of Hydrides: Based on their bonding and structural characteristics, hydrides are broadly classified into three main categories:

Ionic Or Saline Or Saltlike Hydrides

Formation: These hydrides are formed when hydrogen reacts directly with highly electropositive elements, primarily from Group 1 (alkali metals) and Group 2 (alkaline earth metals)** (except beryllium, Be)**. These elements have low electronegativity.

Characteristics:

  • Stoichiometry: Typically stoichiometric, with a formula $MH_n$, where $M$ is the metal and $n$ is its valency (e.g., $NaH$, $CaH_2$).
  • Nature: Crystalline solids, often appearing brittle and having high melting points, similar to ionic compounds.
  • Bonding: Primarily ionic, with hydrogen acting as the hydride ion ($H^-$). The metal atom loses its valence electron(s) to become a positive ion ($M^{n+}$).
  • Properties:
    • They are generally non-conductors in the solid state.
    • However, they conduct electricity when molten or dissolved in suitable solvents (like molten salts or ethers), depositing hydrogen gas at the cathode during electrolysis.
    • They react vigorously with water and acids to liberate hydrogen gas.

Examples:

  • Sodium hydride ($NaH$): $2NaH(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$
  • Calcium hydride ($CaH_2$): $CaH_2(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + H_2(g)$
  • Lithium hydride ($LiH$)
  • Potassium hydride ($KH$)
  • Magnesium hydride ($MgH_2$)
  • Calcium hydride ($CaH_2$)

Special Case - Beryllium Hydride ($BeH_2$): Beryllium lies between ionic and covalent character due to its small size and relatively higher electronegativity. $BeH_2$ is a polymeric covalent compound rather than purely ionic.

Covalent Or Molecular Hydride

Formation: These hydrides are formed when hydrogen reacts with elements of intermediate electronegativity**, mainly from Groups 13 to 17** (non-metals and some metalloids). Hydrogen shares electrons with these elements to form covalent bonds.

Characteristics:

  • Nature: Usually gases, liquids, or low melting point solids.
  • Bonding: Covalent, with discrete molecules.
  • Properties:
    • Generally poor conductors of electricity in any state.
    • Their properties vary widely depending on the electronegativity of the other element and the structure of the molecule.
  • Classification based on electron bonding:
    • Electron-Deficient Hydrides: Formed by elements of Group 13 (e.g., Boron, Aluminum). They have fewer valence electrons than required to form normal covalent bonds, leading to structures with bridging hydrogen atoms (e.g., diborane, $B_2H_6$).
    • Electron-Precise Hydrides: Formed by elements of Group 14 (e.g., Carbon, Silicon). They have the correct number of valence electrons to form normal covalent bonds (e.g., methane $CH_4$, silane $SiH_4$).
    • Electron-Rich Hydrides: Formed by elements of Groups 15, 16, and 17 (e.g., Nitrogen, Oxygen, Halogens). They have lone pairs of electrons on the central atom, influencing their properties like hydrogen bonding. Examples: Ammonia ($NH_3$), Water ($H_2O$), Hydrogen fluoride ($HF$).

Examples: $CH_4$, $SiH_4$, $NH_3$, $PH_3$, $H_2O$, $H_2S$, $HF$, $HCl$, $HBr$, $HI$.

Metallic Or Non-Stoichiometric Hydrides

Formation: These hydrides are formed when hydrogen reacts with transition metals** (elements from Groups 3-12), particularly the d-block and f-block elements.

Characteristics:

  • Non-Stoichiometric: They often do not have fixed ratios of hydrogen to the metal, hence "non-stoichiometric". The composition varies depending on the conditions of formation. For example, $ZrH_{1.75}$, $PdH_x$.
  • Nature: They retain the metallic properties of the parent metal, such as luster, conductivity, and malleability.
  • Bonding: The bonding is complex, often described as hydrogen atoms being absorbed into the metal lattice, forming interstitial compounds. The hydrogen atoms are thought to exist as protons ($H^+$) or ions ($H^-$) within the metal lattice, or simply as H atoms.
  • Properties:
    • Often hard and brittle.
    • Exhibit variable electrical conductivity.
    • Some can store large volumes of hydrogen, making them candidates for hydrogen storage.
    • An exception is palladium ($Pd$), which can absorb large volumes of hydrogen to form $PdH_x$.
  • Deficiency: Some metals, like Iron, Cobalt, and Nickel, do not form true hydrides, although they may absorb hydrogen.

Examples: Hydrides of $Ti, Zr, V, Cr, Mn, Fe, Co, Ni, Pd, Pt$, etc. (e.g., $TiH_{1.5}$, $VH_{0.6}$), Lanthanide and Actinide hydrides.