Periodic Properties of Elements
It says that, as you move down the group, the atomic radius increases. Increased atomic radius lowers the attraction between revolving electrons and the protons inside the nucleus. This makes the atom lose electrons more easily, so it is more electropositive.
This increases the metallic character of the element.
This enables them to more readily make basic oxides. [As metals generally make basic oxides when reaction with oxygen occurs.]
In the Periodic Table, Cesium (Cs) is below Sodium (Na) in Group 1.
Hence, explained my answer.