Alkali metal
The alkali metals are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (sometimes written cesium, although caesium is preferred) (Cs), and francium (Fr). Lithium is the least reactive. Although it was thought before that francium would be the most reactive, though it is very rare, it was more recently predicted that caesium is in fact more reactive than francium due to relativity.