CBSE Class 10 Science 2020 · Paper 31/3/3 · Question 26(b)

(a) Ca (2, 8, 8, 2) has $2$ valence electrons, shown as two dots beside the symbol Ca; O (2, 6) has $6$ valence electrons, shown as six crosses around the symbol O (see the figure). (b) Calcium transfers its $2$ valence electrons to oxygen: $\text{Ca}⟶{\text{Ca}}^{2+}+2e^{-}$ and $\text{O}+2e^{-}⟶{\text{O}}^{2-}$, giving ${\text{Ca}}^{2+}+{\text{O}}^{2-}⟶\text{CaO}$. In dot structures, Ca with two dots and O with six crosses give $[\text{Ca}{]}^{2+}[\text{O}{]}^{2-}$, the oxide ion now carrying eight electrons (see the figure). (c) Calcium ion (${\text{Ca}}^{2+}$) and oxide ion (${\text{O}}^{2-}$). (d) (1) It is a hard, crystalline solid. (2) It has very high melting and boiling points. (3) It is soluble in water, dissolving by reacting with it to form calcium hydroxide, but not in kerosene or petrol. (4) It conducts electricity in the molten state, but not as a solid.