CBSE Class 10 Science 2019 · Paper 31/5/1 · Question 18

(a)(i) Metallic character decreases on moving from left to right in a period: the nuclear charge increases and the atoms become smaller, so the valence electrons are held more tightly and are lost less easily. Example: in the third period, sodium (Na) is more metallic than chlorine (Cl). (ii) Metallic character increases on moving from top to bottom in a group: a new shell is added at each step, so the atoms become larger and the valence electrons, being farther from the nucleus, are lost more easily. Example: potassium (K) is more metallic than sodium (Na). (b) An element of group 14 has four valence electrons, which it shares rather than loses or gains, so the bond in its chloride is covalent. Formula: ${\text{XCl}}_4$. (c) Atomic number $=35-18=17$. Electronic configuration: 2, 8, 7 (K, L, M). With seven valence electrons, X gains one electron to complete its octet, so its valency is 1 (it forms ${\text{X}}^{-}$).