Answer: Lithium (180°C vs Na 98°C, K 63°C, Rb 39°C, Cs 28°C).
- A Lithium (180°C vs Na 98°C, K 63°C, Rb 39°C, Cs 28°C)
- B Sodium, which melts unexpectedly higher than the rest of the group
- C Caesium, despite being the heaviest common alkali metal
- D Rubidium, despite sitting between potassium and caesium
Correct answer: A. Lithium (180°C vs Na 98°C, K 63°C, Rb 39°C, Cs 28°C)
Explanation: Li melts at 180°C; all other alkali metals melt below 100°C. Li's small size means stronger metallic bonding (higher electron density).
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.