Question

Carbon forms millions of compounds whereas silicon, which is just below it in the Periodic Table, forms a few compounds. Explain the reason for the difference in their ability to form compounds.

Hint:

Think about the size of the two atoms and the strength of the bond between two like atoms (catenation).

Answer 1

Step 1: Understanding the Question:
Carbon and silicon are in the same group (Group 14). Both have 4 valence electrons and both show a valency of 4.
Yet carbon makes millions of compounds and silicon makes only a few. We must find what is different about the two atoms.

Step 2: Key Idea: Catenation:
Catenation is the ability of atoms of the same element to join with each other and form long chains, branched chains and rings.
How far catenation goes depends on how strong the bond between two like atoms is.

Step 3: Why carbon forms so many compounds:
Carbon is in the second period, so its atom is very small. The shared pair of electrons in a C-C bond is held close to both nuclei.
This makes the C-C bond very strong (about 350 kJ per mole) and stable, so long chains do not break easily.
Carbon also forms strong bonds with hydrogen, oxygen, nitrogen and halogens, and it can make double and triple bonds. Chains, branches, rings and isomers are all possible, so the number of compounds is huge.

Step 4: Why silicon forms only a few compounds:
Silicon is in the third period, so its atom is much bigger. The shared pair in an Si-Si bond is far from the nuclei and is held weakly.
The Si-Si bond is much weaker (about 225 kJ per mole), so long silicon chains break easily and react with water and air.
Silicon therefore shows very limited catenation and forms only a few compounds.

Final Answer:
Carbon has a small size, so it forms strong and stable C-C bonds and shows extensive catenation. Silicon is larger, forms weak Si-Si bonds and shows very little catenation. \[ \boxed{\text{Small size of C} \Rightarrow \text{strong C-C bonds} \Rightarrow \text{catenation}} \]