Answer: Francium (Fr).
- A Francium (Fr)
- B Lithium
- C Sodium
- D Rubidium
Correct answer: A. Francium (Fr)
Explanation: Francium is the heaviest alkali metal and is intensely radioactive (all isotopes are radioactive, half-life <22 min).
Each s-block metal ion emits a characteristic flame colour because heating excites electrons to higher energy levels; light is emitted as they fall back, with the colour determined by the specific energy gap.
Concept context
Group 1 (alkali metals) and Group 2 (alkaline earth metals). Learn their reactions with water and air, important compounds like NaOH, Na₂CO₃, and CaCO₃, and anomalous behaviour of lithium and beryllium.