Q.Describe how the atomic and ionic radii vary across the first-row transition series from scandium to zinc, and explain the reason for this trend.

Unlike descending a group (where radius increases steadily as a whole new shell is added), moving across the 3d transition series adds each successive electron to an inner ($3d$) orbital, one shell in from the outermost $4s$ electrons that largely determine the atom's size.

From scandium to chromium/manganese, radius decreases: each additional $3d$ electron shields the correspondingly higher nuclear charge only partially from the $4s$ outer electrons, so the effective nuclear charge felt by those outer electrons increases, pulling the electron cloud in.

Through the middle of the series, roughly chromium to nickel, the radius becomes nearly constant. From here on, every new $3d$ electron is added to an orbital that already holds one electron, so the resulting increase in electron-electron repulsion within the $3d$ subshell very nearly cancels the effect of the (still slowly rising) nuclear attraction -- the two effects roughly balance each other out. …

Break the trend into its three phases (fall, plateau, slight rise) and give the competing-effects explanation (rising nuclear charge vs poor d-sh …

Describing the trend as a simple steady decrease across the whole series (missing the plateau and the late rise); attributing the late rise to 'shell change' …