CBSE Class 10 Science 2019 · Paper 31/2/1 · Question 16(a)

(a) Three observations that posed a challenge to Mendeleev’s Periodic Law: 1. Position of hydrogen: hydrogen resembles both the alkali metals (it forms compounds such as $\text{HCl}$ and ${\text{H}}_2\text{O}$, like $\text{NaCl}$ and ${\text{Na}}_2\text{O}$) and the halogens (it exists as diatomic ${\text{H}}_2$, like ${\text{Cl}}_2$), so it could not be given a fixed position. 2. Position of isotopes: isotopes of an element have similar chemical properties but different atomic masses, so an arrangement by atomic mass could not place them. 3. Atomic masses do not increase in a regular manner from one element to the next, so it could not be predicted how many elements were still to be discovered between two elements. (Also accepted: cobalt, $58.9$, had to be placed before nickel, $58.7$.) (b) (i) From left to right in a period, the metallic character decreases, because the effective nuclear charge acting on the valence electrons increases, the atomic size decreases, and the tendency to lose electrons decreases. (ii) From top to bottom in a group, the metallic character increases, because a new shell is added at each step, the atomic size increases, and the valence electrons, being farther from the nucleus, are lost more easily.