UPSC NDA I 2021 General Ability Solved Paper

Concept:Valency is the combining capacity of an element, determined by the number of electrons it can lose, gain, or share to achieve a stable octet. Elements in the same group of the periodic table often have the same valency.Explanation:Check the valency of each element in the given sets:• Sodium ($\mathrm{Na}$, atomic number 11) has valency 1.• Magnesium ($\mathrm{Mg}$, atomic number 12) has valency 2.• Calcium ($\mathrm{Ca}$, atomic number 20) has valency 2.• Aluminium ($\mathrm{Al}$, atomic number 13) has valency 3.• Potassium ($\mathrm{K}$, atomic number 19) has valency 1.• Barium ($\mathrm{Ba}$, atomic number 56) has valency 2.Now examine each option:Option A: $\mathrm{Na}$ (1), $\mathrm{Mg}$ (2), $\mathrm{Ca}$ (2) → not all same.Option B: $\mathrm{Na}$ (1), $\mathrm{Mg}$ (2), $\mathrm{Al}$ (3) → all different.Option C: $\mathrm{Mg}$ (2), $\mathrm{Ca}$ (2), $\mathrm{K}$ (1) → not all same.Option D: $\mathrm{Mg}$ (2), $\mathrm{Ca}$ (2), $\mathrm{Ba}$ (2) → all have valency 2.Thus, only the set $\mathrm{Mg}$, $\mathrm{Ca}$, $\mathrm{Ba}$ has the same valency (2).Answer:D. Mg, Ca, Ba