Why lanthanum are grouped together in periodic table?

Answer 1

The question is not phrased quite right -- lanthanum is a single element. It is the first of a group of 15 elements called "lanthanides", atomic numbers 57-71. They are caused by a pause in the usual periodic progression of electron orbital filling with increased atomic number, where electrons go into a shielded inner 4f orbit instead of the outermost shell. As a result, the chemical properties don't change much from element to element. Thus the lanthanides occur together in nature and are difficult to separate chemically. Most of the lanthanides were first discovered in ore from Ytterby, Sweden, thus they have names derived from that place.

Together with two other lighter elements, scandium and yttrium (which have similar chemical properties), they are known as the "rare earths" although most of them are only semi-rare.