Question

why there is no lanthanoide contraction in yittrium and lanthanum

Answer 1

if you see periodic table , there is no contraction in Yittrium and Lanthanum ,

first of all you understand what is Lanthanoid contraction ?
we know, all elements form by number of subatomic particles ,e.g electrons,protons etc
but this is arrange in such a way that solar system . that is electrons revolve around nucleus , path of revolution is called orbit ,

according to Bohr theory ,
electrons configure in orbit named as K,L, M, N etc.

we know, first orbit maximum no of electrons 2 , in next orbit number of electrons 8, in next 18 , in next 32 or incomplete .
due to suddenly increment of number of electrons in 4th orbit . effective nuclear charge increase , we know, due to effective nuclear charge atom is contract again and again , due to this situation, Lanthanoid contraction occur .

actually , from Yittrium to Lanthanum no 4th orbit exist , so, here Lanthanoid contraction no exist .