why do transition elements have similar properties?

Answer 1

each transition period has a different reason
if u see 4d and 5d period the reason is lanthanoid contraction
and for the 3d the presence of unpaired d electrons should be the reason.

Answer 2

For chemistry, what is important is the NUMBER OF ELECTRONS ON THE OUTER SHELL. If you look at all outer shells (except the first) , the outer shell contains only electrons s and p (maximum 8 electrons).
The elements of transition fill an inner subshell

I give you an example

lets the element Krypton its electronic structure
l is 1s22s22p63s2 3p6

normally the next element should have the structure 1s22s22p63s2 3p6 3d1 or this is not he case the next element K has 1s22s22p63s2 3p6 4s1 after this we have
Ca 1s22s22p63s2 3p6 4s2 and after

We see that there are 10 elements from Sc to Zn which fill the subshell 3d from 3d1 to 3d10

These elements have an OUTER SHELL 4s (I recall you that the outer shell has the greatest number n) and as the chemical properties depend very Little from the inner shell 3d, they have the same properties

Answer 3

dear know thou. that transtional elements show similarties in group due to same no. of ellectron in outermost orbit. furthermore they also exhibit idetical prpperties in periods becaus their differtiating electrons enter in d orbitats of same principal q.number