I... am currently imagining a vest of sodium chainmail. The
best case scenario is that it rapidly oxidises to turn a sort of mucky white colour. The
worst case scenario is that it rains, your armour melts, giving off hydrogen, and then someone lights a match.
Anyway, it's not sodium. But it is a metal that's already been mentioned (amazing though the mental images would be once you got round to plutonium and mercury...).
The straight clue is also a rough translation of the place name (though admittedly very rough).
hS