Q1. [4] § 4.2.1 Saturated and Unsaturated Carbon Compounds § 4.2.2 Chains, Branches and Rings
The combining capacity of various elements depends on the number of valence electrons. Also the reactivity of elements is explained as their tendency to attain a completely filled outer shell, that is, to attain a noble gas configuration. This may be either through gain of electrons or loss of electrons or sharing of electrons.
Answer the following sub-parts based on the given passage about combining capacity and valence electrons.
- (a) An element A has atomic number 16, how will it attain its nearest noble gas configuration?
- (b) Write the number of (i) single and (ii) double covalent bonds in a molecule of butene (C₄H₈).
- (c) Explain the formation of a molecule of ammonia (NH₃), using electron dot structure. (Atomic number of nitrogen is 7)
Previously asked in CBSE board exam
2025 31/5/1 Q37
Generated by claude-sonnet-4-6 · 2026-06-15 06:49 · grounding stimulus
Model Answer
(a) Element A has atomic number 16 (Sulphur). Its electronic configuration is 2, 8, 6. It has 6 valence electrons, so it needs to gain 2 electrons to attain the nearest noble gas configuration (of Argon, 2, 8, 8).
(b) Butene: C₄H₈
- (i) Single bonds: 9 (C–C single bonds + C–H bonds)
- (ii) Double bonds: 1 (one C=C double bond)
(c) Nitrogen (atomic number 7) has electronic configuration 2, 5 — it has 5 valence electrons and needs 3 more. Each hydrogen atom has 1 valence electron. Nitrogen shares 3 electrons with three hydrogen atoms (one from each H), forming 3 N–H single covalent bonds. The remaining one pair of electrons on nitrogen is a lone pair. Thus NH₃ molecule is formed by sharing of electrons.
$$\text{N has 1 lone pair + 3 bonding pairs → NH}_3$$
Source: Chapter – Carbon and its Compounds / Periodic Classification of Elements, Valence Electrons and Chemical Bonding
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Explanation
- (a): Identify valence electrons from atomic number → state gain/loss → name the noble gas attained.
- (b): In butene (CH₃–CH=CH–CH₃ or CH₂=CH–CH₂–CH₃), count carefully: 1 double bond and the remaining C–C and C–H bonds are all single (9 total).
- (c): Examiners expect you to mention electron dot structure, number of electrons shared, lone pair on N, and that it is covalent bonding. Drawing the dot structure on paper earns marks.
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