CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 18

(a) From the electronic configuration, find the number of electrons in the outermost shell (the valence electrons). An atom loses, gains or shares electrons to complete its octet, so the valency equals the number of valence electrons when this is 4 or less, and 8 minus the number of valence electrons when this is more than 4. For atomic number 9 the configuration is 2, 7: the atom needs 1 electron to complete its octet, so its valency is $8-7=1$. (b) Configurations: A 2, 2; B 2, 7; C 2, 8, 4; D 2, 8, 8, 1; E 2, 8, 8, 2. (i) D (2, 8, 8, 1) has one electron in its outermost shell. (ii) A and E belong to the same group (group 2), because both have 2 electrons in their outermost shell (and the same valency, 2). (iii) D and E are in the fourth period, and D has the bigger atomic radius. Across a period the nuclear charge increases while electrons are added to the same shell, so the electrons are pulled closer to the nucleus and the atomic radius decreases; D comes before E, so D is larger.