Hydrides
Ionic, Covalent, Metallic and the Notorious Hydride Gap.
Under the right conditions, Dihydrogen ($H_2$) combines with almost every element in the periodic table (except noble gases) to form binary compounds called Hydrides. Depending on the electronegativity and size of the element it combines with, hydrides are classified into three strict categories: Ionic, Covalent, and Metallic.
Figure 1: Structural comparison of the three fundamental classes of Hydrides.
1. Ionic or Saline Hydrides
Formed primarily by the highly electropositive s-block elements (Alkali and Alkaline Earth metals). Because these metals are so eager to lose electrons, they force Hydrogen to accept an electron, forming the Hydride ion ($H^-$).
*Exception:* Beryllium ($Be$) and Magnesium ($Mg$) are less electropositive. Their hydrides ($BeH_2$ and $MgH_2$) are significantly covalent and polymeric in nature.
Key Properties:
- Structure: Crystalline, non-volatile, and non-conducting in the solid state.
- The Electrolysis Trap: In the molten (liquid) state, they conduct electricity. However, during electrolysis, Hydrogen gas is liberated at the ANODE (positive electrode). This definitively proves the existence of the negatively charged $H^-$ ion.
Anode Reaction: $2H^-_{(melt)} \rightarrow H_{2(g)} + 2e^-$ - Reaction with Water: They react violently and explosively with water to form metal hydroxides and release $H_2$ gas. Because of this, solid $CaH_2$ is commercially known as Hydrolith (a portable source of hydrogen).
$NaH_{(s)} + H_2O_{(aq)} \rightarrow NaOH_{(aq)} + H_{2(g)}$
2. Covalent or Molecular Hydrides
Formed by the p-block elements. Hydrogen shares electrons via standard covalent bonds to form discrete molecules. Because the p-block spans a wide range of groups, covalent hydrides are strictly sub-classified into three categories based on the number of valence electrons.
A. Electron-Deficient Hydrides (Group 13)
These hydrides do not have enough valence electrons to form normal, 2-center-2-electron bonds for all atoms. They must share electrons in unusual ways (like the 3c-2e Banana Bonds).
- Example: Diborane ($B_2H_6$).
- Chemical Nature: Because they lack a full octet, they act as powerful Lewis Acids (electron-pair acceptors).
B. Electron-Precise Hydrides (Group 14)
These elements have exactly the right number of valence electrons to form normal covalent bonds, completing their octet with zero lone pairs remaining.
- Example: Methane ($CH_4$), Silane ($SiH_4$).
- Structure: They usually have a perfect tetrahedral geometry.
C. Electron-Rich Hydrides (Groups 15, 16, 17)
These elements have more valence electrons than needed for bonding. The excess electrons reside on the central atom as lone pairs.
- Examples: $NH_3$ (1 lone pair), $H_2O$ (2 lone pairs), $HF$ (3 lone pairs).
- Chemical Nature: Because they possess lone pairs, they act as Lewis Bases (electron-pair donors).
- The Boiling Point Trap: The presence of highly electronegative atoms ($N, O, F$) with lone pairs leads to extensive intermolecular Hydrogen Bonding. This causes $NH_3$, $H_2O$, and $HF$ to have unusually high boiling and melting points compared to the heavier hydrides in their respective groups.
3. Metallic or Interstitial Hydrides
Formed by d-block and f-block (transition and inner-transition) elements. In these compounds, hydrogen atoms are so small that they slip into the microscopic empty spaces (interstitial voids) between the large metal atoms in the crystal lattice.
Not all d-block elements form hydrides. Elements of Groups 7, 8, and 9 absolutely DO NOT form hydrides. This massive region in the periodic table is famously known as the Hydride Gap.
(In Group 6, only Chromium forms a hydride: $CrH$).
Key Properties:
- Non-Stoichiometric: Because hydrogen just fills random voids, the compounds do not follow the law of constant composition. Formulas are often fractional, e.g., $LaH_{2.87}$, $TiH_{1.5-1.8}$, $PdH_{0.6-0.8}$.
- Metallic Properties: Unlike ionic or covalent hydrides, metallic hydrides retain the properties of the parent metal. They are hard, possess metallic luster, and conduct electricity and heat (though slightly less efficiently than the pure metal).
- Hydrogen Storage (Occlusion): Certain metals like Palladium ($Pd$) and Platinum ($Pt$) can accommodate enormous volumes of hydrogen gas in their lattices (a process called occlusion). Because the hydrogen can be released reversibly by heating, these metals are intensely researched as solid-state Hydrogen Storage media for clean-energy vehicles.
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