All alkali metals have one valence electron, $n{s}^1$, outside the noble gas core except :

All alkali metals have one valence electron, $n{s}^1$, outside the noble gas core except Sodium, francium and cesium.

Each alkali metal  has one valence electron that is not a part of the noble gas core. These elements are the most electropositive metals due to the loosely held s-electron in their outermost valence shell. To produce monovalent M+ ions, they easily lose their electrons.