Question

On what ground you can say that scandium ( z = 21) is a transition element but zinc ( z= 30 is not

Answer 1

Transition elements are those which have an unpaired electron in the ground state or any of the excited state
Sc has unpaired d electon but zn doesn't so it is not considered as transition element.

Answer 2

scandium outer configuration is 3d^2 4s^1 as incompletely filled d orbital it have chance to transition where as in zinc d orbital is completely filled so the outer configuration is 3d^10 4s^2 thus it does not exhibit transition properties