CBSE Class 10 Science 2020 · Paper 31/2/3 · Question 25(b)

(a) Aluminium (atomic number $13$, electronic configuration $2,8,3$) loses its three valence electrons to form the aluminium ion, and chlorine (atomic number $17$, electronic configuration $2,8,7$) gains one electron to form the chloride ion; one aluminium atom transfers one electron to each of three chlorine atoms (see the electron-dot diagram): $\text{Al}(2,8,3)⟶{\text{Al}}^{3+}(2,8)+3e^{-}$ $\text{Cl}(2,8,7)+e^{-}⟶{\text{Cl}}^{-}(2,8,8)$ $\text{Al}+3\text{Cl}⟶{\text{Al}}^{3+}+3{\text{Cl}}^{-}({\text{AlCl}}_3)$ Both ions then have a completely filled outermost shell. (b) Formula: ${\text{AlCl}}_3$ (aluminium chloride). Nature: an ionic (electrovalent) compound, because it is formed by the transfer of electrons. (c) Any three characteristics of compounds formed by transfer of electrons (ionic compounds), with reasons: (i) They are solids and somewhat hard, because of the strong force of attraction between the positive and negative ions. (ii) They have high melting and boiling points, because a considerable amount of energy is needed to break the strong inter-ionic attraction. (iii) They conduct electricity in the molten state (and in aqueous solution), because the ions are then free to move; they do not conduct in the solid state, where the ions are held in fixed positions. (They are also generally soluble in water and insoluble in kerosene and petrol; other correct characteristics with reasons also earn credit.)