CBSE Class 10 Science 2019 · Paper 31/4/3 · Question 19

(a) Criteria: (1) the atomic masses of the elements, arranged in increasing order; (2) the similarity of their chemical properties, judged from the formulae of their oxides and hydrides, so that similar elements fall in the same group. Mendeleev’s Periodic Law: The properties of elements are a periodic function of their atomic masses. Hydrogen: It resembles the alkali metals (group 1), having one electron in its outermost shell and forming $\text{HCl}$, ${\text{H}}_2\text{O}$ and ${\text{H}}_2\text{S}$, with formulae like $\text{NaCl}$, ${\text{Na}}_2\text{O}$ and ${\text{Na}}_2\text{S}$; it also resembles the halogens (group 17), existing as diatomic molecules (${\text{H}}_2$, like ${\text{Cl}}_2$). Since it resembles both groups, no fixed position could be given to it. (b) (i) From left to right in a period, the atomic size decreases: the nuclear charge increases while the electrons are added to the same shell, so the electrons are pulled closer to the nucleus. (ii) Down a group, the atomic size increases: a new shell is added at each step, so the outermost electrons are farther from the nucleus.