CBSE Class 10 Science 2020 · Paper 31/4/1 · Question 26

(a) (i) Gold has a bright metallic lustre, and it is highly malleable and ductile, so it can be worked into fine ornaments (it is also very unreactive, so it does not tarnish easily). (ii) Silver and copper. (iii) Gallium and caesium. (b) Calcium $(2,8,8,2)$ loses its two valence electrons to form ${\text{Ca}}^{2+}$ $(2,8,8)$; oxygen $(2,6)$ gains these two electrons to form ${\text{O}}^{2-}$ $(2,8)$. In the electron-dot structure the two dots of calcium move to oxygen (six crosses), giving $[\text{Ca}{]}^{2+}$ and $[\text{O}{]}^{2-}$ with a complete octet. The oppositely charged ions are held together by strong electrostatic attraction, forming the ionic compound $\text{CaO}$.