Answer: First ionisation energy.
- A First ionisation energy
- B Atomic radius
- C Density
- D Reactivity with water
Correct answer: A. First ionisation energy
Explanation: Down Group 1, atomic radius increases (more shells), which reduces nuclear attraction on valence electrons, lowering ionisation energy.
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.