Question

why the elements of 6th period,in total 15 elements are known as Lanthanum series and 7th period as Actinide series...???

Answer 1

First of all, it's lathonoids and actinoids.
So, basically they are radioactive elements with presence of f orbitals. So, lathonoids are called so because of lathonoid contractions and similarly actinoids are called so because of actinoid contractions.