CBSE Class 10 Science 2018 · Paper 31/1/C · Question 17(a)

Across a period the number of shells stays the same, while the nuclear charge increases by one at each step and each extra electron enters the same outermost shell. Hence (taking Period 3, Na to Ar, as the example): (a) Valency first increases from 1 to 4 and then decreases to 0 (Na 1, Mg 2, Al 3, Si 4, P 3, S 2, Cl 1, Ar 0). Justification: the number of valence electrons increases from 1 to 8; for 1 to 4 valence electrons the valency equals that number, and for 5 to 8 it equals 8 minus that number. (b) Atomic radius decreases (Na $186$ pm to Cl $99$ pm). Justification: the nuclear charge increases while electrons are added to the same shell, so the electrons are pulled closer to the nucleus. (c) Metallic character decreases and non-metallic character increases (Na, Mg, Al metals; Si metalloid; P, S, Cl non-metals). Justification: as the effective nuclear charge increases and the atoms become smaller, the tendency to lose electrons decreases and the tendency to gain electrons increases. (d) Electronegative character increases. Justification: the smaller size and greater nuclear charge increase the attraction of the nucleus for electrons, so the tendency to gain electrons increases. (e) The oxides change from basic to amphoteric to acidic (${\text{Na}}_2\text{O}$, $\text{MgO}$ basic; ${\text{Al}}_2{\text{O}}_3$ amphoteric; ${\text{SiO}}_2$, ${\text{P}}_4{\text{O}}_{10}$, ${\text{SO}}_3$, ${\text{Cl}}_2{\text{O}}_7$ acidic). Justification: metals form basic oxides and non-metals form acidic oxides, and the elements change from metals to non-metals across the period.