CBSE Class 10 Science 2025: question 28

Hints

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

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.

1. Electronic configurations: Na (Z = 11): 2, 8, 1, so it has 1 valence electron. O (Z = 8): 2, 6, so it has 6 valence electrons.

2. (i) Electron-dot structure of sodium: $\mathrm{Na}\cdot$ (one dot).

   ½ mark

3. (ii) Electron-dot structure of oxygen: the symbol O with six dots around it (two pairs and two single dots).

   ½ mark

4. Formation of sodium oxide: each of two Na atoms transfers its one electron (shown by arrows) to one O atom. O gains 2 electrons and completes its octet.
   $2\mathrm{Na}\cdot +\mathrm{O}(6\text{ dots})\to 2\mathrm{N}{\mathrm{a}}^{+}+[\mathrm{O}(8\text{ electrons}){]}^{2-}$, i.e. $(\mathrm{N}{\mathrm{a}}^{+}{)}_2[\mathrm{O}{]}^{2-}$ = $\mathrm{N}{\mathrm{a}}_2\mathrm{O}$.

   1 mark

5. Anion: oxide ion, ${\mathrm{O}}^{2-}$.

   ½ mark

6. Cation: sodium ion, $\mathrm{N}{\mathrm{a}}^{+}$.

   ½ mark

Answer: Na· and O with 6 dots; 2Na· + O → (Na⁺)₂[O]²⁻ (Na₂O); anion O²⁻, cation Na⁺.

Revise the chapter

• Chapter: Metals and Non-metals (notes, formulas, where marks are lost and every past question)
• Topic: Ionic Compounds: Formation and Properties