Explain the variations in atomic radii of transition elements along the period.

The atomic radii of transition elements in a period decreases with the increase in atomic number and the decrease becomes small after middle of the series. However, the atomic radii are higher than $p$-block elements but smaller than $s$-block elements.

In the beginning of the series, the atomic radii of the elements decreases with the increase in atomic number because of increase in nuclear charge. The attraction between the nucleus and outermost electrons $(4 \mathrm{~s})$ increases which results in decrease of atomic radii from $Sc$ to $Cr$.

In the middle of the series, the sheilding effect of $d$-electrons and the nuclear attraction are counter balanced with the increases in atomic number and so the atomic radii is almost steady in middle of each series.

Ex. : from $\mathrm{Cr}$ to $\mathrm{Cu}$ in case of first transition series, the atomic radii is nearly same.

At the end of the series, the electron-electron repulsions in d-orbitals gets dominant over the nuclear attraction with the increase in nuclear charge. These repulsions will expand the electron cloud which results in increase of atomic radii. Ex. : The atomic radius of zinc is greater than copper.