Dear Student,

lanthanide series is a series comprising of rare earth elements which are 15 of the metallic chemical elements of atomic numbers 57-71 from lanthanum through lutetium.

Cesium is known as the most electropositive element as it is known to lose its single valence electron and forms ionic bonds with nearly all the inorganic and organic anions.

Hello aspirant

The lanthanides are generally considered to be elements with atomic numbers 58-71 (lanthanum to lutetium). The lanthanide series is the group of elements in which the 4f sublevel is being filled.

As cesium as low electronic affinity it is most electropositive.

Hope this helps. All the best.