Hydrides — ionic, covalent, interstitial: Question bank

Before the traps, three anchors so no symbol below is used unexplained:

True or false — justify

Whether ionic hydrides conduct electricity in the solid state

False — the ions are locked in a rigid lattice; only when molten do

M^{+}

and

H^{-}

become mobile and carry charge.

H⁻ is a smaller ion than a neutral H atom because it has fewer electrons

False —

H^{-}

has one more electron than H, so electron–electron repulsion swells it; the two electrons feel only one proton, making

H^{-}

large and easily deformed.

All Group 2 elements form ionic hydrides

False — BeH₂ is polymeric/covalent because

B e^{2 +}

is tiny with huge charge density and heavily polarizes

H^{-}

(Fajans' rules); the rest (Mg→Ba) are essentially ionic.

MgH₂ is purely ionic like NaH

False (mostly true) — it is predominantly ionic but carries noticeable covalent character, because

M g^{2 +}

still has enough charge density to distort the hydride ion somewhat.

In an ionic hydride, hydrogen is the oxidising agent

False —

H^{-}

is a reducing agent; it readily donates its extra electron (

H^{-} \to H + e^{-}

), so it reduces other species.

Down Group 15 the hydride bond angle stays near 107° (as in NH₃)

False — it drops sharply toward ~93–94° (

P H_{3}, A s H_{3}

) because the larger central atom overlaps poorly with H 1s, so bonds sit almost along pure p-orbitals.

H₂O boils higher than H₂S mainly because water is heavier

False — water is lighter (18 vs 34 g/mol); the real cause is hydrogen bonding in

H_{2} O

, absent in

H_{2} S

Interstitial hydrides always obey a fixed whole-number formula

False — they are typically non-stoichiometric (e.g.

TiH_{1.7}

PdH_{0.6}

) because H fills only some of the available lattice gaps depending on conditions.

A covalent hydride cannot be a strong acid

False —

H F

and

H C l

are covalent hydrides yet acidic; high partner electronegativity weakens the

H - X

bond and stabilises the released

X^{-}

Ionic hydrides are denser than the parent metal

True — inserting the small, space-filling

H^{-}

into the packing raises mass per volume, so e.g. LiH is denser than Li metal.

Why questions

Why does the lattice energy term decide whether an ionic hydride forms at all?

The formation is uphill through sublimation + ionisation +

H_{2}

dissociation; only the large negative lattice energy (e.g. −808 kJ/mol for NaH) can outweigh those costs and make

Δ H_{f}

negative.

Why is

H^{-}

described as "isoelectronic with helium"?

By gaining one electron it reaches the

1 s^{2}

configuration — the same electron count as He — which is why it's a very poor Brønsted acid and hence a strong base.

Why does bond angle fall in the order

C H_{4} > N H_{3} > H_{2} O

Lone pairs rise 0 → 1 → 2 down that list; each lone pair sits closer to the nucleus and repels bond pairs more, squeezing the angle from 109.5° → 107° → 104.5°.

Why do interstitial hydrides keep a metallic look and conduct electricity?

Hydrogen only occupies gaps in the metal lattice without stripping the metal's sea of delocalised electrons, so metallic conduction (and lustre) survive.

Why is

H_{2}

gas the tell-tale product when a hydride meets water?

The lattice's

H^{-}

(oxidation state −1) meets water's

H^{+}

(oxidation state +1); they combine to

H_{2}

(oxidation state 0), one H reducing the other — a self-redox that liberates gas.

Why does acidity increase

N H_{3} < H_{2} O < H F

across period 2?

Rising partner electronegativity (N < O < F) both weakens the

H - X

bond and better stabilises the leftover

X^{-}

anion, so the proton departs more readily.

Edge cases

Where does hydrogen sit that makes it hardest to classify — near a metalloid like B?

With boron (

χ = 2.0

, essentially equal to H's 2.1) neither full transfer nor clean sharing works, giving electron-deficient covalent hydrides with exotic 3c-2e bonds.

What happens to the classification of a "hydride" if the partner is exactly as electronegative as hydrogen?

Electron transfer has no driving force, so bonding is purely covalent/shared — the boundary case that separates ionic from covalent behaviour.

Is HF ionic because F is so electronegative that it grabs the electron fully?

No — even with the biggest

Δ χ

among period-2 hydrides, HF stays covalent (polar); it forms a discrete molecule and only ionises in water, unlike a lattice-based ionic hydride.

Can a single metal form a hydride of variable H content depending on temperature/pressure?

Yes — interstitial hydrides like palladium's absorb variable H (e.g.

PdH_{x}

up to

x \approx 0.6

) because filling of lattice gaps is condition-dependent, not fixed.

What is the degenerate case of "zero lone pairs" in the bond-angle trend, and what angle results?

C H_{4}

— with no lone pairs the four bond pairs spread to the ideal tetrahedral 109.5°, the undistorted reference for the whole series.

Recall Self-test before you leave

Which single property (state) distinguishes molten ionic hydrides from solid ones electrically? ::: Molten conducts (mobile ions); solid does not. Name the bond type in the bridges of diborane. ::: 3-centre-2-electron bonds. One reason BeH₂ is not ionic. ::: $B e^{2 +}$ 's high charge density polarizes $H^{-}$ (Fajans), forcing covalent/polymeric bonding.

Question bank — Hydrides — ionic, covalent, interstitial

True or false — justify

Spot the error

Why questions

Edge cases