Chapter 3 · Metals and Non-metals

NCERT Class 10 Science Chapter 3 Intext Questions (Page 49)

All 2 questions of NCERT Class 10 Science Chapter 3 Intext Questions (Page 49) — Metals and Non-metals — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   1. (i)Write the ​electron-dot structures for ​sodium, oxygen and magnesium.
   2. (ii)Show the formation ​of ${\text{Na}}_2\text{O}$ and $\text{MgO}$ by ​the transfer of electrons.
   3. (iii)What ​are the ions present in ​these compounds?
   Show answer

   Sodium and magnesium give electrons to oxygen, forming ${\text{Na}}_2\text{O}$ with ${\text{Na}}^{+}$ and ${\text{O}}^{2-}$, and $\text{MgO}$ with ${\text{Mg}}^{2+}$ and ${\text{O}}^{2-}$.

   (i) Electron-dot structures: Na with one dot (2,8,1), O with six dots (2,6), Mg with two dots (2,8,2), as in the figure. (ii) ${\text{Na}}_2\text{O}$: two sodium atoms each lose one electron and one oxygen atom gains both, $2\text{Na}\to 2{\text{Na}}^{+}+2e^{-}$ and $\text{O}+2e^{-}\to {\text{O}}^{2-}$, giving $({\text{Na}}^{+}{)}_2{\text{O}}^{2-}$, that is ${\text{Na}}_2\text{O}$. $\text{MgO}$: one magnesium atom loses two electrons and one oxygen atom gains them, $\text{Mg}\to {\text{Mg}}^{2+}+2e^{-}$ and $\text{O}+2e^{-}\to {\text{O}}^{2-}$, giving ${\text{Mg}}^{2+}{\text{O}}^{2-}$, that is $\text{MgO}$. (iii) ${\text{Na}}_2\text{O}$ contains ${\text{Na}}^{+}$ and ${\text{O}}^{2-}$ ions (two ${\text{Na}}^{+}$ for each ${\text{O}}^{2-}$); $\text{MgO}$ contains ${\text{Mg}}^{2+}$ and ${\text{O}}^{2-}$ ions (one of each).

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2. Q2
   Why ​do ​ionic compounds ​have high ​melting ​points?
   Show answer

   Strong electrostatic attraction between the ions needs a large amount of heat energy to overcome, so ionic compounds melt at high temperatures.

   Ionic compounds are made of positively and negatively charged ions held together by strong electrostatic forces of attraction in a rigid structure. To melt such a solid these strong inter-ionic attractions must be overcome so that the ions can move freely, and this requires a considerable amount of heat energy. Hence ionic compounds have high melting points (for example, NaCl melts at 1074 K and CaO at 2850 K).

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