Answer: Caesium.
- A Caesium
- B Lithium
- C Sodium
- D Potassium
Correct answer: A. Caesium
Explanation: Caesium is the softest alkali metal (and among all metals); it can be cut with a knife.
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.