Hydrogen — Hydrides: Ionic, Covalent, and Metallic

• Classify hydrides into ionic, covalent, and metallic (interstitial) types with examples
• Explain why ionic hydrides contain H⁻ and act as strong reducing agents
• Compare covalent hydrides of p-block elements and explain their volatile, molecular nature
• Describe metallic/interstitial hydrides and their non-stoichiometric compositions
• Analyse bond angle trends (NH₃ > H₂O > H₂S) using lone pair repulsion and electronegativity
• Compare reducing character trends in Group 16 and Group 17 hydrides

Level 1 — Foundations

Classification of Hydrides

Ionic Hydrides

• Formed when s-block metals (Na, K, Ca, Ba, etc.) react with H₂: $2\text{Na} + \text{H}_2 \rightarrow 2\text{NaH}$ $\text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2$

• Contain H⁻ (hydride ion) — hydrogen has gained one electron

• High melting points (ionic lattice): NaH mp = 800°C

• React violently with water → H₂ gas evolved: $\text{NaH} + \text{H}_2\text{O} \rightarrow \text{NaOH} + \text{H}_2 \uparrow$

• Strong reducing agents (H⁻ donates electrons easily): $\text{LiAlH}_4 \text{ (lithium aluminium hydride) — used in organic chemistry to reduce aldehydes, ketones}$

• BeH₂ and MgH₂ are exceptions — they have polymeric covalent character due to high charge density of Be²⁺/Mg²⁺

Covalent Hydrides

• p-block elements (Groups 14–17) form molecular hydrides
• Properties depend on electronegativity and lone pairs:

Metallic/Interstitial Hydrides

• d-block metals absorb H₂ into their crystal lattice (interstitial positions)
• Non-stoichiometric: composition varies — PdH₀.₆, TiH₁.₇
• Metallic lustre and conductivity retained
• Useful for hydrogen storage (hydrogen economy)
• Pd can absorb up to 900× its own volume of H₂
• Note: Group 7 and 8 metals (Mn, Fe, Co, Ni) do NOT readily form hydrides — this is the "hydride gap"

Bond Angle Trends in Hydrides

NH₃ (107°) > H₂O (104.5°): Water has 2 lone pairs vs NH₃'s 1 lone pair → greater lp–lp repulsion compresses bond angle further in H₂O.

NH₃ (107°) >> PH₃ (93.3°): P is larger and less electronegative → P–H bonding pairs are more diffuse and farther from P → less bp–bp repulsion → smaller bond angle.

Level 2 — JEE Depth

Why Bond Angles Decrease: Lone Pair Repulsion + Electronegativity

VSEPR predicts repulsion order: lp–lp > lp–bp > bp–bp

For NH₃ vs PH₃:

• N is small and highly electronegative (3.0) → bond pairs held close to N → strong bp–bp repulsion → angle = 107°
• P is larger and less electronegative (2.1) → bond pairs drawn towards H → less bp–bp repulsion → angle = 93.3°

The s-character of the lone pair also matters (Bent's Rule): with lower electronegativity of central atom, lone pair occupies orbital with more s-character (more stable), pushing bond pairs into orbitals with more p-character → smaller angles.

For H₂O vs H₂S:

• O (electronegativity 3.5, small) → 2 lp close to O → bond angle = 104.5°
• S (electronegativity 2.5, large) → 2 lp more diffuse → bp drawn to H → angle = 92.1°

Reducing Character of Hydrides

Group 16 (H₂O < H₂S < H₂Se < H₂Te):

Reducing character increases down the group. The E–H bond strength decreases down the group (O–H > S–H > Se–H > Te–H) — weaker bonds are easier to break → easier to lose H (or electrons) → stronger reducing agent.

$\text{Bond energies: O–H (459 kJ/mol) > S–H (363 kJ/mol) > Se–H (276 kJ/mol) > Te–H (238 kJ/mol)}$

H₂O is the weakest reducing agent (O–H bond very strong). H₂Te is the strongest.

Group 17 (HF < HCl < HBr < HI):

Similarly, reducing character increases HF < HCl < HBr < HI: $\text{Bond energies: H–F (569 kJ/mol) > H–Cl (431 kJ/mol) > H–Br (364 kJ/mol) > H–I (297 kJ/mol)}$

HI is the strongest reducing agent among hydrogen halides — it reduces H₂SO₄ to H₂S: $8\text{HI} + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S} + 4\text{I}_2 + 4\text{H}_2\text{O}$

HCl reduces H₂SO₄ only to SO₂: $2\text{HCl} + \text{H}_2\text{SO}_4 \rightarrow \text{SO}_2 + \text{Cl}_2 + 2\text{H}_2\text{O}$

Polymeric Structure of BeH₂ and AlH₃

BeH₂ is not ionic (Be²⁺ has too high charge density — polarises H⁻ to become H). Instead, it forms a polymeric chain:

• Each Be bridges two H atoms through 3-centre 2-electron (3c-2e) bonds
• Similar to electron-deficient boron compounds

$\text{...Be–H–Be–H–Be–H...}$ $(\text{polymeric chain with bridging H atoms})$

This is why BeH₂ and MgH₂ behave more like covalent polymers despite being Group 2 hydrides.

Palladium Hydride — Hydrogen Storage

$\text{Pd} + x\text{H}_2 \rightarrow \text{PdH}_{2x} \quad (x \approx 0.3 \text{ at room temp, up to } 0.6 \text{ under pressure})$

H atoms occupy octahedral interstitial sites in the FCC Pd lattice. The process is reversible — Pd releases H₂ on heating. This is the basis of Pd-membrane hydrogen purification and solid-state hydrogen storage research.

Example 1: Classify the following as ionic, covalent, or metallic hydrides and give one distinguishing property of each: (a) NaH (b) SiH₄ (c) TiH₁.₇ (d) CaH₂

(a) NaH — IONIC HYDRIDE
  - Na is a Group 1 alkali metal (s-block)
  - Contains H⁻ ion; Na⁺H⁻
  - High melting point (~800°C); reacts with water to give H₂
  - Strong reducing agent

(b) SiH₄ (Silane) — COVALENT HYDRIDE
  - Si is a Group 14 p-block element
  - Molecular; covalent Si–H bonds
  - Volatile gas (bp = −112°C); no ionic character
  - Used in semiconductor deposition (CVD)

(c) TiH₁.₇ — METALLIC/INTERSTITIAL HYDRIDE
  - Ti is a d-block (transition) metal
  - Non-stoichiometric (1.7 ≠ simple integer)
  - H atoms trapped in Ti crystal lattice interstitially
  - Retains metallic properties (lustre, conductivity)

(d) CaH₂ — IONIC HYDRIDE
  - Ca is a Group 2 alkaline earth metal (s-block)
  - Contains Ca²⁺ and 2H⁻
  - Reacts vigorously with water: CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂↑
  - Used as a drying agent and H₂ source

Example 2: PH₃ has a bond angle of 93.3° while NH₃ has 107°. Explain. Also predict the bond angle of AsH₃ and justify whether it will be greater than, equal to, or less than PH₃.

NH₃ vs PH₃:
  Both have 1 lone pair and 3 bond pairs on the central atom.
  Ideal tetrahedral angle = 109.5°; lone pair compresses this.

  N: small atom, high electronegativity (3.0)
  → bond pairs held close to N → strong bp–bp repulsion
  → bond angle = 107° (close to tetrahedral)

  P: larger atom, lower electronegativity (2.1)
  → bond pairs pulled towards H, away from P
  → reduced bp–bp repulsion near P
  → bond angle = 93.3° (close to 90°, approaching p-orbital geometry)

AsH₃:
  As is below P in Group 15; even larger, even less electronegative (2.0)
  → bond pairs even more displaced towards H
  → bp–bp repulsion near As even smaller
  → bond angle < 93.3°

  Experimental: AsH₃ bond angle = 91.8° ✓ (less than PH₃)

Conclusion: Bond angles in Group 15 hydrides: NH₃ (107°) > PH₃ (93.3°) > AsH₃ (91.8°) > SbH₃ (91.3°)

• Q1: Write the reaction of CaH₂ with water. What type of hydride is it and what is the role of CaH₂ in this reaction?
• Q2: Arrange Group 17 hydrides in order of increasing reducing power: HCl, HBr, HI, HF.
• Q3: Why is PdH₀.₆ considered a non-stoichiometric compound?
• Q4: Explain why H₂O has a larger bond angle than H₂S despite both having 2 lone pairs on the central atom.
• Stretch: Q5: LiH, NaH, KH are ionic hydrides. Yet BeH₂ and MgH₂, despite being Group 2 compounds, are NOT truly ionic. Explain using polarisation/Fajans' rules. Also explain why LiH is more covalent than NaH by the same argument. Predict which is a stronger reducing agent: LiAlH₄ or NaBH₄, giving a reason based on hydride ion donor ability.

NCERT Chapter 9 link: Hydrogen — Section 9.6 (Hydrides — Types and Properties), Section 9.4 (Properties of Hydrogen)

Exam connections: JEE Mains tests classification of hydrides and bond angle trends regularly. JEE Advanced asks mechanistic explanations for reducing character trends with thermochemical justification. NEET: type identification and water/ammonia comparison. Board: classification table with examples and properties.

Study strategy: Draw a 3-column classification table first (ionic/covalent/metallic), fill in examples from s-, p-, d-blocks. Then separately memorise bond angle trends as two rows: Group 15 (NH₃ > PH₃ > AsH₃) and Group 16 (H₂O > H₂S). Link reducing character to bond energy — lower bond energy = stronger reducer.

• Hydride Classification Sorter: Drag-and-drop element cards into ionic/covalent/metallic buckets; system checks classification; reveals ionic radius, electronegativity, and group number as hints if stuck.
• Bond Angle Explorer: 3D VSEPR model builder — add lone pairs one at a time to central atom and observe bond angle compression; compare NH₃, PH₃, H₂O, H₂S side by side with numerical angle display.
• Reducing Power Ladder: Interactive electrochemical series for hydrides; add reducing agents one by one; observe which can reduce H₂SO₄ to SO₂ vs H₂S; connect to bond energy data.

• "NaH reacts with water to give H₂ gas. In this reaction, is the H in NaH being oxidised or reduced? Track the oxidation state of H from NaH to H₂ to H₂O. Can you tell me what NaH's role is?"
• "If I give you a mystery hydride with a non-integer formula like MH₁.₄, what type of hydride is it? What does a non-integer ratio tell you about how hydrogen is incorporated into the structure?"
• "Group 17 hydrides: HF has the strongest bond yet it's the weakest reducing agent — HI has the weakest bond but strongest reducer. This seems backwards from what you might expect about 'strong bonds = stable compound = poor reducer.' Can you reconcile this — is stability the same as reducing power?"

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• Hydrogen storage for green energy: Metallic hydrides (TiFeH, LaNi₅H₆) are candidate materials for solid-state hydrogen storage tanks in hydrogen fuel cell vehicles — engineers balance H₂ absorption capacity, weight, and desorption temperature to design practical tanks.
• NaBH₄ in portable power: Sodium borohydride (NaBH₄) releases H₂ on contact with water catalytically — this compact hydride system powers portable fuel cells for drones and military electronics; chemists optimise the catalyst to control H₂ release rate.
• LiAlH₄ in pharmaceutical synthesis: Ionic hydride H⁻ from LiAlH₄ is a nucleophile that reduces C=O, C=N, NO₂ groups — used industrially to manufacture drugs like ibuprofen and antihistamines; understanding H⁻ as a nucleophile bridges inorganic hydride chemistry to organic synthesis.

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