Answer: Lithium.
- A Lithium
- B Sodium
- C Potassium
- D Caesium
Correct answer: A. Lithium
Explanation: Lithium is the first and smallest alkali metal; atomic radius increases down the group as shells are added.
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.