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

Electronic configurations: Na (11) 2, 8, 1; Mg (12) 2, 8, 2; Al (13) 2, 8, 3. (a) Aluminium. These metals lose their valence electrons, so their valencies equal their numbers of valence electrons: Na $1$, Mg $2$, Al $3$. Aluminium, with three valence electrons, has the highest valency, $3$. (b) Sodium. All three atoms have their valence electrons in the same (third) shell, but the nuclear charge rises from $+11$ to $+12$ to $+13$; the greater pull draws the electrons closer, so the atomic radius decreases from Na to Mg to Al. Sodium has the largest atomic radius. (c) Sodium. These metals react by losing their valence electrons. Sodium has only one, in the largest atom and under the weakest pull of the nucleus, so it loses it most easily and is the most reactive. (Sodium reacts vigorously with cold water, magnesium only with hot water, and aluminium only with steam.)