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Science · 2025 · 4 marks
CBSE 2025 · Region 5 · Set 1 · Q37
The combining capacity of various elements depends on the number of valence electrons. Also the reactivity of elements is explained as their tendency…
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 $\displaystyle 16$, 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 $\displaystyle \left(\mathrm{C}_{4} \mathrm{H}_{8}\right)$.
(c)(A)
Explain the formation of a molecule of ammonia $\displaystyle \left(\mathrm{NH}_{3}\right)$, using electron dot structure. (Atomic number of nitrogen is $\displaystyle 7$)
OR
(c)
(B)
Why does carbon share its valence electrons with other atoms of carbon or with atoms of other elements ?
Marking-scheme solution
(a) It shall gain or share $\displaystyle 2$ electrons to attain its nearest noble gas configuration.
(b) (i) The number of single covalent bonds- $\displaystyle 10$
(ii) The number of double covalent bonds-$\displaystyle 1$
(c) (A)
(B) Carbon cannot gain or lose $\displaystyle 4$ electrons to complete its octet as a large amount of energy is involved. /
(i) It could gain four electrons forming \(\displaystyle \mathrm{C}^{4-}\) anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons.
(ii) It could lose four electrons forming \(\displaystyle \mathrm{C}^{4+}\) cation. But it would require a large amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding on to just two electrons.
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CBSE Class 10 Science past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.