This.
(+1 to whoever correctly identifies it)
Cesium or Rubidium
Since it has a silvery gold color, my best guess would be cesium.
Swallowing a chunk of sodium would probably hurt more as it would fester and burn before exploding. The cesium reaction with water is so violent it tends to not react all of the cesium with the target. Since a good portion of the Cesium gets blown away before it can react.
Aka, Sodium gives you more boom.