CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 10

P (atomic number 20): electronic configuration 2, 8, 8, 2. It has 4 shells, so it is in period 4, and 2 valence electrons, so it is in group 2 (P is calcium). Q (atomic number 17): electronic configuration 2, 8, 7. It has 3 shells, so it is in period 3, and 7 valence electrons, so it is in group 17 (Q is chlorine). Formula: P loses its 2 valence electrons (valency 2) and each Q atom gains 1 electron (valency 1), so one P atom combines with two Q atoms. The compound is ${\text{PQ}}_2$, i.e. ${\text{CaCl}}_2$, an ionic compound made of ${\text{P}}^{2+}$ and ${\text{Q}}^{-}$ ions in the ratio 1 : 2.