CBSE Class 10 Science 2022 · Paper 31(B)/6 · Question 10

Calcium ($Z=20$) has the electronic configuration $2,8,8,2$: four shells, so period 4, and two valence electrons, so group 2. (a) Calcium is a metal. It has two valence electrons, which it loses readily to form the positive ion ${\text{Ca}}^{2+}$, and it lies in group 2, towards the left of the periodic table, among metals such as sodium and magnesium. (b) (i) The atomic radius of calcium is smaller than that of potassium. Both are in period 4 (potassium is $2,8,8,1$), and the larger nuclear charge of calcium ($+20$ against $+19$) pulls the electrons of the same outermost shell closer. (ii) The atomic radius of calcium is larger than that of iron. Iron ($2,8,14,2$) is also in period 4 but further to the right, and its larger nuclear charge ($+26$) pulls its shells closer; atomic size decreases from left to right along a period. (c) The oxide is $\text{CaO}$. Calcium and oxygen both have valency $2$, so ${\text{Ca}}^{2+}$ and ${\text{O}}^{2-}$ combine in the ratio $1:1$.