Question bank — Catenation and the diversity of organic molecules

False — tetravalency lets a chain branch and extend, but the chain only survives if each $Si-Si$ link is strong and unreactive; silicon fails both (weak links ≈222 kJ/mol, attacked via empty $3d$ orbitals).

False — strength is necessary but not sufficient; the chains also need to be kinetically inert (high activation barrier to attack by water/air), which comes from carbon lacking valence $d$-orbitals and lone pairs.

False — bond strength tracks orbital overlap, which is best when atoms are small and close; a bigger atom means a longer bond and poorer overlap, so $Si-Si$ is weaker than $C-C$.

False — it means any self-bonding pattern: straight chains, branched chains, and rings (cyclohexane, benzene) are all catenation.

False — one formula can hide many structural isomers ($C_4{H}_{10}$ gives 2, $C_5{H}_{12}$ gives 3, $C_{20}{H}_{42}$ gives 366319), which is the mathematical face of organic diversity.

True — moving down group 14, atoms get bigger, bonds get longer, overlap worsens: mean bond enthalpies 348 > 222 > 188 kJ/mol (gas phase, 298 K), so catenation weakens down the group.

False — the reason is kinetic, not thermodynamic: combustion of alkanes is very exothermic, but the activation barrier to attack at room temperature is high, so they sit inert until ignited.

True in spirit — both come from carbon being small: short distance means good end-on overlap ($\sigma$) and good sideways $2p-2p$ overlap ($\pi$), so $C=C$ and $C\equiv C$ are stable.

Error — short silanes (like $Si{H}_4$, $S{i}_2{H}_6$) do exist; the correct claim is that long silanes are not stable, because 19+ weak $Si-Si$ links snap and the chain is oxidised/hydrolysed easily.

Error — carbon has only 4 valence electrons, fewer than silicon's shells; the strength comes from carbon's small size giving a short bond with excellent overlap, not from electron count.

Error — the causal arrow is reversed: having 4 valence electrons (and needing 4 bonds) enables catenation and branching; catenation does not cause the valence.

Error — two separate reasons: weakness lowers the cost of breaking, but the attack pathway (the low activation barrier) exists because silicon has empty, accessible $3d$ orbitals for a nucleophile like $O{H}^{-}$ to use; carbon has no such orbitals.

Error — a ring is carbon bonded back to itself, which is exactly catenation; rings are a major branch of organic diversity alongside chains.

Error — branching changes shape and boiling point, but it does not automatically make an isomer "more stable"; stability depends on the specific structure, and this claim confuses counting isomers with ranking them.

Error — small size gives good $\pi$ overlap, but the exceptional stability comes from aromatic delocalisation: the six $2p$ electrons spread over the whole ring, lowering the energy by a large resonance energy (≈150 kJ/mol for benzene) beyond a hypothetical fixed-bond ring.

Carbon's radius (≈77 pm) is much smaller than silicon's (≈117 pm), so bonding electrons sit close to the nucleus and the two orbitals overlap strongly at a short distance.

After using two of its four bonds to link to neighbouring carbons, carbon has two spare bonds left for H, O, N, halogens or a branch — it is never "used up."

Benzene has 6 delocalised $\pi$ electrons that satisfy Hückel's rule ($4n+2$ with $n=1$), giving large aromatic stabilisation; an $N_6$ ring is riddled with lone-pair repulsion and weak $N-N$ single bonds (≈163 kJ/mol vs $C-C$ 348), so it is unstable/explosive and does not persist.

Spreading the $\pi$ electrons over all six carbons (Hückel's $4n+2=6$) lowers the total energy by the resonance energy — benzene sits ≈150 kJ/mol below the energy expected for a ring of three fixed double bonds, an extra bonus on top of ordinary bond strength.

Each added carbon multiplies the ways of arranging the skeleton (chain length, branch position), a combinatorial blow-up — from 2 isomers at $C_4$ to hundreds of thousands by $C_{20}$.

Atoms get progressively larger (C→Si→Ge→Sn), bonds get longer, overlap worsens, and heavier atoms gain accessible $d$-orbitals that invite attack — so self-bonding becomes both weaker and more reactive.

Carbon's valence shell (the $2s,2p$ level) has no $d$-orbitals to accept an incoming electron pair, so a nucleophile has no low-energy orbital to attack into — the activation barrier stays high.

No — catenation requires at least one carbon–carbon bond; methane has only $C-H$ bonds, so the smallest catenated molecule is ethane ($C_2{H}_6$).

Not bond strength (each $C-C$ stays strong) but practical factors — entropy, synthesis difficulty, and folding — yet chains as long as DNA backbones and polymers show carbon can go essentially indefinitely.

The one without accessible $d$-orbitals, because equal strength ties the thermodynamics, so the deciding factor is kinetic inertness (a higher activation barrier to attack) — no easy pathway means longer-surviving chains.

Cyclopropane ($C_3$) is the smallest; its 60° internal angles force bond angles far from the ideal ~109.5°, so poor overlap makes it strained and reactive — a reminder that geometry, not just self-bonding, governs ring stability.

No — with only 2 bonds each carbon could form a chain but never branch or attach many functional groups, collapsing most of the diversity; tetravalency and catenation must work together.