CBSE Class 10 Science 2017 · Paper 31/2/3 · Question 10

(a) Electronic configuration: 2, 8, 2 (K = 2, L = 8, M = 2). The atom has 2 valence electrons, which it loses, so its valency is 2. (The element is magnesium, Mg.) (b) It is less reactive than calcium. Calcium ($Z=20$) has the configuration 2, 8, 8, 2: both elements have 2 valence electrons and are in group 2, with calcium below magnesium. Calcium's atom has one more shell, so its valence electrons are farther from the nucleus and held less strongly; calcium loses them more easily and is therefore more reactive. (c) It is a metal: it loses its 2 valence electrons to form a positive ion, ${\text{Mg}}^{2+}$. (d) The formula of its oxide is $\text{MgO}$.