What is the primary factor determining whether a hydride is ionic, covalent, or metallic?

The primary factor is the electronegativity difference between hydrogen and the element it bonds with. If the element is highly electropositive (low electronegativity, like Group 1 and 2 metals), the bond will be predominantly ionic, forming $H^-$ ions. If the electronegativity difference is small or the element is more electronegative than hydrogen (like p-block elements), the bond will be covalent. For transition and inner transition metals, hydrogen occupies interstitial sites, leading to metallic hydrides.

Why are ionic hydrides strong reducing agents?

Ionic hydrides contain the hydride ion, $H^-$, which has a strong tendency to donate its extra electron to achieve a stable $1s^0$ configuration (like Helium). This strong electron-donating ability makes $H^-$ a powerful reducing agent, capable of reducing other species by providing electrons. For example, it can reduce metal oxides or even water.

How do electron-deficient, electron-precise, and electron-rich covalent hydrides differ?

These classifications are based on the number of valence electrons available for bonding around the central atom. Electron-deficient hydrides (e.g., $B_2H_6$) have fewer electrons than needed for conventional bonds, often forming multi-center bonds. Electron-precise hydrides (e.g., $CH_4$) have exactly enough electrons to form conventional bonds and complete the central atom's octet. Electron-rich hydrides (e.g., $NH_3$, $H_2O$) have surplus electrons as lone pairs on the central atom, which can participate in hydrogen bonding or act as Lewis bases.

What is the 'hydride gap' in the periodic table?

The 'hydride gap' refers to the observation that elements of Group 7, 8, and 9 (Mn, Fe, Co, Ni, etc.) in the d-block do not form hydrides under normal conditions. This is an exception to the general trend of transition metals forming metallic hydrides. The reasons are complex, often attributed to unfavorable thermodynamic or kinetic factors for hydrogen absorption in these specific metals.

Can metallic hydrides conduct electricity? If so, how do their conductivity compare to the parent metals?

Yes, metallic hydrides generally retain their metallic character and can conduct electricity. This is because the hydrogen atoms occupy interstitial sites within the metal lattice, and the valence electrons of the metal remain delocalized, forming an 'electron sea'. However, their electrical conductivity is often lower than that of the parent metals. The absorbed hydrogen atoms can introduce scattering centers or alter the electronic band structure, slightly impeding electron flow.

Why are BeH2 and MgH2 considered covalent, despite Be and Mg being Group 2 metals?

While most Group 2 metals form ionic hydrides, Beryllium and Magnesium are exceptions. Beryllium has a very small size and high polarizing power, leading to significant covalent character in its bonds. Magnesium also exhibits some covalent character in its hydride. Both $BeH_2$ and $MgH_2$ exist as polymeric structures with bridging hydrogen atoms, consistent with covalent bonding rather than discrete $M^{2+}H_2^-$ ionic units. This is a crucial point for NEET aspirants.

Ionic, Covalent and Metallic Hydrides

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Hydrides are binary compounds formed between hydrogen and other elements. Based on the nature of bonding and their physical and chemical properties, hydrides are broadly classified into three main categories: ionic (saline or salt-like) hydrides, covalent (molecular) hydrides, and metallic (interstitial) hydrides. This classification primarily depends on the electronegativity difference between hy…

Quick Summary

Hydrides are binary compounds of hydrogen with other elements, classified into three main types based on bonding and properties. Ionic hydrides (Group 1 & 2 metals like NaH, CaH2) are salt-like, formed by electron transfer to hydrogen ( $H^-$ ).

They are strong reducing agents, react violently with water to produce $H_2$ and a base, and are non-conductive solids but conductive in molten state. Covalent hydrides (p-block elements like $CH_4$ , $NH_3$ , $H_2O$ ) are molecular, formed by electron sharing.

They are classified as electron-deficient (e.g., $B_2H_6$ ), electron-precise (e.g., $CH_4$ ), or electron-rich (e.g., $NH_3$ ) based on electron count around the central atom, influencing their geometry and intermolecular forces.

Their properties vary widely from acidic to basic. Metallic hydrides (d-block & f-block elements like $TiH_x$ , $PdH_x$ ) are interstitial, non-stoichiometric compounds where hydrogen occupies voids in the metal lattice.

They retain metallic properties, are hard solids, and are important for hydrogen storage. The 'hydride gap' refers to the absence of hydrides for Group 7, 8, 9 metals.

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Key Concepts

Reactivity of Ionic Hydrides with Water

Ionic hydrides, such as sodium hydride (NaH), contain the highly reactive hydride ion ( $H^-$ ). When these…

Electron Count in Covalent Hydrides and its Implications

The classification of covalent hydrides into electron-deficient, electron-precise, and electron-rich…

Non-stoichiometry in Metallic Hydrides

Unlike ionic and covalent hydrides which typically have fixed, simple whole-number ratios of elements, many…

Ionic Hydrides — Group 1 & 2 metals (

NaH, CaH_2

H^-

ion. Crystalline solids. High MP. Conductive when molten. React vigorously with

H_2O \rightarrow H_2 + \text{base}

. Strong reducing agents.

Covalent Hydrides — p-block elements (

CH_4, NH_3, H_2O

). Molecular. Gases/liquids/low MP solids. Non-conductive.

- Electron-deficient: < 8 e- on central atom ( $B_2H_6$ ). Lewis acids. - Electron-precise: 8 e- on central atom ( $CH_4$ ). - Electron-rich: > 8 e- on central atom (lone pairs, $NH_3, H_2O$ ). Lewis bases, H-bonding.

Metallic Hydrides — d & f-block metals (

TiH_x, PdH_x

). Interstitial. Non-stoichiometric. Retain metallic properties (conductivity). Hydrogen storage.

Exceptions —

BeH_2, MgH_2

are covalent. Hydride gap: Group 7, 8, 9 metals do not form hydrides.

To remember the three types of hydrides and their key features, think: 'I C M'

Ionic: Interacts with water to make Ionic base +

H_2

. In Group 1 & 2. Ionically bonded (

H^-

Covalent: Can be electron-deficient, precise, or rich. Central atom has lone pairs or not. Conducts no electricity. Common in p-block.

Metallic: Many d & f-block elements. Mainly non-stoichiometric. Maintains metallic properties. Makes good hydrogen storage.