Periodicity Of Valency

Valency is the combining capacity of an element, and with respect to oxygen it follows a regular periodic pattern tied to the number of valence electrons. Across a period, the valency with respect to oxygen increases with the group number, since more valence electrons become available for bonding. For example, in period three the oxides progress as Na_2O (valency 1), MgO (2), Al_2O_3 (3), SiO_2 (4), P_4O_10 (5), SO_3 (6), and Cl_2O_7 (7), so the valency rises in step with the group number up to a maximum. Thus the valency increases across the period and is directly related to the group. The option of steady decrease reverses the trend. The option of constancy ignores the changing valence-electron count. The option of no pattern contradicts the clear regularity. This periodicity of valency is a standard NCERT classification concept. Carefully relating the data to the governing principle ensures the reasoning remains valid even when the numbers or species in the question are changed. This concept also bridges to p-Block Elements and Chemical Bonding and Molecular Structure, so mastering it strengthens performance on linked questions from those topics as well. Plausibility check: the highest oxide of chlorine, Cl_2O_7, shows valency seven, matching its group number, confirming the increasing trend.