Discuss the three types of covalent hydrides.

Write Brief Answer · Q2

Q. Discuss the three types of covalent hydrides.

Tamil Nadu DgeTextbookSubjectiveImportance★★★★★

6% · 4/64 Questions

✓ Free question

Step 1. Electron-precise hydrides -- the central atom's valence electrons exactly match what is needed for bonds to every hydrogen attached, with nothing left as a lone pair and no shortfall either. Example: carbon (4 valence e⁻) forms exactly 4 bonds in $CH_4$ , using all 4 electrons.

Step 2. Electron-deficient hydrides -- the central atom does not have enough valence electrons to form ordinary two-centre, two-electron bonds to every hydrogen present. Example: boron (3 valence e⁻) in $B_2H_6$ (diborane) cannot form four normal B-H bonds per boron, so the molecule instead uses unusual three-centre, two-electron "banana" bridge bonds to hold the bridging hydrogens in place.

Step 3. Electron-rich hydrides -- the central atom has one or more lone pairs of electrons remaining after bonding to all its hydrogens. Example: nitrogen (5 valence e⁻) uses only 3 in bonding to three H atoms in $NH_3$ , leaving one lone pair; oxygen (6 valence e⁻) uses only 2 in bonding to two H atoms in $H_2O$ , leaving two lone pairs.

✓Final answer

Electron-precise hydrides (e.g. CH₄, C₂H₆, SiH₄, GeH₄) use exactly enough valence electrons for all their H bonds with none left over; electron-deficient hydrides (e.g. B₂H₆) lack enough electrons for conventional bonds to every H and use bridge bonding instead; electron-rich hydrides (e.g. NH₃, H₂O) have one or more lone pairs left over after bonding to all their H atoms.

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2019Set ANN1 mark
$Q. Which among the following is a molecular hydride?$
›Reveal solutionSolution
Hydrides are classified as ionic (saline), molecular (covalent), or metallic (interstitial), based on how hydrogen bonds with the other element; NH3 is the molecular one here.

Hydrogen forms three broad classes of binary hydrides depending on the electronegativity/nature of the partner element:

Ionic (saline) hydrides — formed by highly electropositive s-block metals (Li, Na, K, Ca, etc.), where H exists as H⁻. LiH is an example.

Molecular (covalent) hydrides — formed by p-block non-metals/metalloids that share electrons with H in discrete covalent molecules. NH3 (nitrogen trihydride) is a classic molecular hydride, along with CH4, H2O, HF, etc.

Metallic (interstitial) hydrides — formed by d- and f-block transition/inner-transition metals, where H atoms occupy interstitial sites in the metal lattice, often giving non-stoichiometric formulas. CrH and LaH2.87 (non-integral H:La ratio) are both metallic/interstitial hydrides.

So among the four options, only NH3 is a discrete covalent molecule — the molecular hydride.

✓Final answer
$NH3 is the molecular hydride (LiH is ionic/saline; CrH and LaH2.87 are metallic/interstitial hydrides).$
CBSE 2018Set ANNUAL1 markMCQ
$Q. The most stable hydride of the following:$
(a) AsH3
(b) NH3
(c) BiH3
(d) PH3
›Reveal solutionSolution

Among AsH3, NH3, BiH3 and PH3, NH3 is the most thermally stable hydride because bond dissociation enthalpy decreases down the group.

AsH3, NH3, BiH3 and PH3 are hydrides of Group 15 elements (N, P, As, Bi). Going down the group, atomic size increases, so the bond formed between the central atom and hydrogen (M-H bond) becomes longer and weaker, i.e., the bond dissociation enthalpy decreases: N-H > P-H > As-H > Bi-H. A hydride with stronger, higher-energy bonds is more thermally stable (harder to decompose on heating). Since nitrogen is the smallest atom in the group, NH3 has the strongest N-H bonds and hence is the most stable hydride, while BiH3 (largest central atom) is the least stable and decomposes most readily.

✓Final answer
$The correct option is (b) NH3, the most stable hydride.$

←Previous questionQ1 — Explain why hydrogen is not placed with the halogen in the periodic ta…→Next questionQ3 — Predict which of the following hydrides is a gas on a solid (a) HCl (b…

Q.Discuss the three types of covalent hydrides.

Concept understanding — Covalent Hydrides — Electron-deficient, Electron-precise, Electron-rich

Q.Discuss the three types of covalent hydrides.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days