CBSE Class 10 Science 2019 · Paper 31/4/2 · Question 18

(a) Limitations of Newlands' Law of Octaves: (i) The law held only up to calcium; after calcium, every eighth element did not have properties similar to the first. (ii) It could not give hydrogen a proper position (hydrogen was placed in the same column as fluorine and chlorine). Other correct limitations also earn credit, for example: Newlands assumed that only $56$ elements existed, so elements discovered later did not fit; and he placed two elements in one slot (cobalt and nickel) and grouped unlike elements together. (b) Electronic configurations: A (atomic number $20$): K L M N = $2,8,8,2$ (calcium, Ca); B (atomic number $17$): K L M = $2,8,7$ (chlorine, Cl). Molecular formula: A has valency $2$ and B has valency $1$, so the compound is ${\text{AB}}_2$, that is, ${\text{CaCl}}_2$. Nature: The compound is neutral. It is the salt of a strong acid (hydrochloric acid, HCl) and a strong base (calcium hydroxide, ${\text{Ca(OH)}}_2$), so its solution in water has a pH of about $7$.