CBSE Class 10 Science 2020 · Paper 31/5/3 · Question 26(b)

(a) Electronic configurations: Al (13) is 2, 8, 3 and O (8) is 2, 6. Each Al atom loses its 3 valence electrons to form ${\text{Al}}^{3+}$ (2, 8), and each O atom gains 2 electrons to form ${\text{O}}^{2-}$ (2, 8). Since $2\times 3=3\times 2=6$, two Al atoms transfer 6 electrons to three O atoms, as in the electron-dot figure, giving $[{\text{Al}}^{3+}{]}_2[{\text{O}}^{2-}{]}_3$, that is ${\text{Al}}_2{\text{O}}_3$; the oppositely charged ions are held together by strong electrostatic attraction. (b) (i) Hydrogen gas is evolved: bubbles of a colourless gas are given off, and the gas burns with a pop sound. (ii) The oxide dissolves, forming a salt and water (with aluminium oxide, sodium aluminate and water). (iii) No reaction takes place; no change is seen. (iv) Brisk effervescence: carbon dioxide gas is given off, which turns lime water milky.