Q1. [2] deep thorough-understanding
In methane (CH₄), carbon forms four single bonds with four hydrogen atoms. A student suggests that carbon could instead form two double bonds, each shared with two hydrogen atoms. Explain why this is not possible, using the electronic configuration of hydrogen in your answer.
Generated by claude-sonnet-4-6 · 2026-06-26 01:08 · grounding rag
Model Answer
A double bond requires both atoms involved to share two pairs of electrons. Hydrogen has only one electron in its outermost shell (electronic configuration: 1), so it can share only one electron — forming one single bond. It cannot form a double bond. Therefore, carbon cannot form two double bonds with hydrogen atoms in CH₄.
Source: Chapter 4, Section 4.2 & 4.2.1
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Explanation
- The key point examiners expect: hydrogen's valency is 1 because it has only 1 electron and needs just 1 more to complete its shell (duplet). It can only ever form one covalent bond.
- A double bond needs each atom to contribute 2 electrons to the shared pairs — hydrogen simply doesn't have enough electrons to do this.
- Always link the electronic configuration (2,1 → only 1 electron available for sharing) to the conclusion. Writing "hydrogen has valency 1" alone without explaining why may lose a mark.