‌‌20‌Ca=[Ar]4s2 ‌‌21‌Sc=[Ar]4s2,3d1 ‌‌22‌Ti=[Ar]4s2,3d2 As d-orbital have diffused shape, hence their electron shields nuclear charge upto lesser extent. Hence, due to increase in effective nuclear charge (Z‌eff ‌), atomic size decrease, in the following order, Ca>Sc>Ti