Ionic, Covalent and Metallic Hydrides — Revision Notes

⚡ 30-Second Revision

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.

2-Minute Revision

Hydrides are binary compounds of hydrogen, classified into three types. Ionic hydrides form with highly electropositive metals (Group 1 & 2). They contain the $H^-$ ion, are salt-like solids, and react vigorously with water to produce $H_2$ gas and a strong base (e.

g., $NaH + H_2O \rightarrow NaOH + H_2$). They are strong reducing agents and conduct electricity in the molten state. Covalent hydrides form with p-block elements and some s-block (Be, B). They are molecular, with properties varying based on electron count: electron-deficient (e.

g., $B_2H_6$, Lewis acids), electron-precise (e.g., $CH_4$), and electron-rich (e.g., $NH_3, H_2O$, Lewis bases, strong H-bonding). Their physical states range from gases to low-melting solids. Metallic hydrides are formed by d- and f-block elements.

They are interstitial, often non-stoichiometric (e.g., $TiH_{1.7}$), and retain metallic conductivity. They are important for hydrogen storage. Remember key exceptions: $BeH_2$ and $MgH_2$ are covalent, and the 'hydride gap' exists for Group 7, 8, 9 metals.

5-Minute Revision

Let's quickly review the three main types of hydrides: ionic, covalent, and metallic, focusing on their defining characteristics for NEET.

1. Ionic Hydrides (Saline Hydrides):

Formation: — Formed by Group 1 (alkali metals) and Group 2 (alkaline earth metals, except Be and Mg) with hydrogen. Examples: $LiH, NaH, CaH_2$.
Bonding: — Predominantly ionic, involving electron transfer from metal to hydrogen, forming $M^+H^-$ ions.
Properties: — Crystalline, non-volatile solids with high melting points. Non-conductive in solid state but conduct electricity in molten state (due to mobile $H^-$ ions).
Reactivity: — Powerful reducing agents. React vigorously with water to produce hydrogen gas and a strong base. For example, $NaH(s) + H_2O(l) \rightarrow NaOH(aq) + H_2(g)$. This reaction is a key identifier.

2. Covalent Hydrides (Molecular Hydrides):

Formation: — Formed by p-block elements (e.g., C, N, O, F, Si, P, S, Cl) and some s-block elements (Be, B). Examples: $CH_4, NH_3, H_2O, HCl$.
Bonding: — Covalent, involving electron sharing.
Classification by Electron Count:

* Electron-deficient: Central atom has < 8 valence electrons (e.g., $B_2H_6$). Act as Lewis acids (electron acceptors). * Electron-precise: Central atom has exactly 8 valence electrons (e.g., $CH_4, SiH_4$). * Electron-rich: Central atom has lone pairs (e.g., $NH_3, H_2O, HF$). Act as Lewis bases (electron donors) and exhibit hydrogen bonding, leading to higher boiling points.

Properties: — Can be gases, liquids, or low-melting solids. Generally non-conductive.

3. Metallic Hydrides (Interstitial Hydrides):

Formation: — Formed by many d-block and f-block elements (transition and inner transition metals). Examples: $TiH_{1.7}, PdH_{0.7}, LaNi_5H_6$.
Bonding: — Hydrogen atoms occupy interstitial sites within the metal lattice. Bonding is complex, retaining metallic character.
Properties: — Often non-stoichiometric (variable composition). Retain metallic luster, hardness, and electrical conductivity (though often reduced). Solid at room temperature.
Applications: — Important for hydrogen storage and as catalysts (e.g., Pd).
Hydride Gap: — Group 7, 8, 9 metals (Mn, Fe, Co, Ni) do not form hydrides under normal conditions.

Key Exceptions to Remember: $BeH_2$ and $MgH_2$ are covalent (polymeric) hydrides, not ionic, due to the small size and high polarizing power of Be and Mg.

Prelims Revision Notes

1
Hydrides Definition — Binary compounds of hydrogen with other elements.
2
Classification Basis — Electronegativity difference between hydrogen and the other element.

I. Ionic (Saline) Hydrides:

* Elements: Group 1 (Li, Na, K, Rb, Cs) and Group 2 (Ca, Sr, Ba) metals. (Exception: Be, Mg form covalent). * Bonding: Predominantly ionic ($M^+H^-$). * Physical State: Crystalline solids, high melting points, non-volatile. * Conductivity: Non-conductive in solid state; good conductors in molten state (due to mobile $H^-$ ions). * Reactivity: Strong reducing agents. React vigorously with water: $MH(s) + H_2O(l) \rightarrow MOH(aq) + H_2(g)$. * Example: $NaH, CaH_2$.

II. Covalent (Molecular) Hydrides:

* Elements: p-block elements (C, N, O, F, Si, P, S, Cl) and Be, B. * Bonding: Covalent (electron sharing). * Physical State: Gases, liquids, or low-melting solids. * Conductivity: Non-conductive (insulators).

* Classification by Electron Count: * Electron-deficient: Central atom has < 8 valence electrons (e.g., $B_2H_6$). Act as Lewis acids. * Electron-precise: Central atom has exactly 8 valence electrons (e.

g., $CH_4, SiH_4$). * Electron-rich: Central atom has lone pairs (e.g., $NH_3, H_2O, HF$). Act as Lewis bases, exhibit hydrogen bonding (higher BP). * Reactivity: Varies (acidic like $HCl$, basic like $NH_3$, neutral like $CH_4$).

III. Metallic (Interstitial) Hydrides:

* Elements: d-block and f-block elements (transition and inner transition metals). * Bonding: Hydrogen occupies interstitial sites in metal lattice; complex bonding. * Stoichiometry: Often non-stoichiometric (e.

g., $TiH_{1.7}, PdH_{0.7}$). Variable composition. * Physical State: Hard, greyish-black solids. Retain metallic luster. * Conductivity: Conductive (though often less than parent metal). * Applications: Hydrogen storage, catalysts.

* Hydride Gap: Group 7, 8, 9 elements (Mn, Fe, Co, Ni) do not form hydrides under normal conditions.

Crucial Exceptions: $BeH_2$ and $MgH_2$ are polymeric covalent hydrides, not ionic.

Vyyuha Quick Recall

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.