Down group I from lithium to caesium, the reactivity of the metals
- A. decreases, because atomic radius increases
- B. stays constant
- C. decreases, because ionisation energy falls
- D. increases, because the outer electron is further from the nucleus and more easily lost
Explanation
Each element down the group has an extra shell, so the single outer electron is further from the nucleus and better shielded, ionisation energy falls and the metal loses that electron more readily. This is why potassium ignites the hydrogen it releases from water while lithium merely fizzes. In the halogens the same reasoning gives the opposite trend, since there reactivity depends on gaining an electron.