CBSE Class 10 Science 2017 · Paper 531 · Question 8

Groups are the vertical columns of the Modern Periodic Table (18 in all); the elements of a group have the same number of valence electrons (in the main groups, helium apart) and similar chemical properties. Periods are the horizontal rows (7 in all); the elements of a period have the same number of shells, and across the main groups their number of valence electrons increases from left to right. (i) Atomic size: A is larger than B. Across a period the nuclear charge increases while the electrons enter the same shell, so atomic size decreases. (ii) Metallic character: A is more metallic than B. A loses its single valence electron more readily, and metallic character decreases across a period. (iii) Valencies in forming oxides: A has a valency of 1 and B a valency of 2, so their oxides are ${\text{A}}_2\text{O}$ and $\text{BO}$. (iv) Formulae of their chlorides: $\text{ACl}$ and ${\text{BCl}}_2$.