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. (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) (A) Explain the formation of a molecule of ammonia (NH3), using electron dot structure. (Atomic number of nitrogen is 7) (c) (B) Why does carbon share its valence electrons with other atoms of carbon or with atoms of other elements?

ScienceCBSE Class 10 · Board exam 2025, set 31/5/1 · 4 marks · Case-based

CBSE Class 10 Science 2025: question 37

Set in the CBSE Class 10 board exam of 2025, paper 31/5/1, section E, question 37, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q37 · 4 marks · Case-basedCompetency-basedCarbon and its Compounds

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.

(a)
An element A has atomic number 16, how will it attain its nearest noble gas configuration?
1 mark
(b)
Write the number of (i) single and (ii) double covalent bonds in a molecule of butene (C4H8).
1 mark
(c)
(A) Explain the formation of a molecule of ammonia (NH3), using electron dot structure. (Atomic number of nitrogen is 7)
2 marks
OR
(c)
(B) Why does carbon share its valence electrons with other atoms of carbon or with atoms of other elements?
2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Write the electronic configuration first and count the valence electrons.

Hint 2

For butene, draw the full structure and count every C–H and C–C bond separately.

Hint 3

Ask how many electrons each atom needs to complete its octet (or duplet for H), and whether gaining or losing that many is easy.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(a) The electronic configuration of A (Z = 16) is 2, 8, 6, with 6 valence electrons. It gains 2 electrons (forming $A^{2 -}$ ) or shares 2 electrons to reach the configuration 2, 8, 8 of the nearest noble gas, argon.
1 mark
(b) Butene is $C H_{2} = C H - C H_{2} - C H_{3}$ . (i) Single covalent bonds = 8 C–H + 2 C–C = 10. (ii) Double covalent bonds = 1 (the C=C bond).
1 mark
(c)(A) Nitrogen (2, 5) has 5 valence electrons and needs 3 more to complete its octet. Each hydrogen has 1 electron and needs 1 more for a duplet. So N shares one electron pair with each of three H atoms, forming three N–H single covalent bonds.
1 mark
Electron dot structure: Write N in the centre with three H atoms around it. Put one shared pair (one dot from N, one cross from H) between N and each H, and one lone pair on N. N now has 8 electrons around it and each H has 2: $H : N \cdot\cdot : H$ with the third H below N.
1 mark
(c)(B) Carbon (2, 4) has 4 valence electrons. It cannot easily gain 4 electrons to form $C^{4 -}$ , because a nucleus with only 6 protons cannot hold 10 electrons.
1 mark
It also cannot easily lose 4 electrons to form $C^{4 +}$ , because removing 4 electrons needs a very large amount of energy. Therefore carbon completes its octet by sharing its 4 valence electrons, forming covalent bonds.
1 mark

Answer: (a) Gains or shares 2 electrons to reach argon's configuration 2, 8, 8; (b) (i) 10 single bonds, (ii) 1 double bond; (c)(A) N shares one electron pair with each of 3 H atoms, giving 3 N–H bonds and 1 lone pair on N; (c)(B) forming $C^{4 -}$ or $C^{4 +}$ is energetically unfavourable, so carbon shares electrons.

Where the marks are

States that A (2, 8, 6) gains or shares 2 electrons to reach the noble gas configuration (1 mark)
10 single covalent bonds in butene (½ mark)
1 double covalent bond in butene (½ mark)
(c)(A) Correct electron dot structure of $N H_{3}$ showing three shared pairs and one lone pair (2 marks)
(c)(B) Explains that gaining 4 electrons is unfavourable ( $C^{4 -}$ : 6 protons cannot hold 10 electrons) (1 mark)
(c)(B) Explains that losing 4 electrons needs too much energy ( $C^{4 +}$ ), so carbon shares (1 mark)

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Chapter: Carbon and its Compounds (notes, formulas, where marks are lost and every past question)

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CBSE Class 10 Science 2025: question 37

Hints

Step-by-step solution

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