Q.Arrange Cr, Mn, Fe, Co, Ni, Cu and Zn in order of increasing metallic radius, and explain why the radius rises again towards the end of the series.

Across the middle-to-late part of the first-row transition series, metallic radius first flattens out (Cr through Ni are all within about 3 pm of one another, roughly $115$-$118$ pm) and then rises noticeably by the time zinc is reached ($\approx 125$ pm).

Comparing Cr, Mn, Fe, Co and Ni: these five sit in the "plateau" region of the trend described earlier in this chapter, where the increase in nuclear charge across each step is very nearly cancelled by the increase in electron-electron repulsion within the increasingly occupied $3d$ subshell, so their radii are all close to one another, with nickel typically the smallest of the five.

Moving on to copper and especially zinc, the radius begins to increase. By copper, the $3d$ subshell is completely filled ($3d^{10}4s^1$); by zinc, both the $3d^{10}$ and $4s^2$ subshells are completely filled. In each case, adding a further electron to an already substantially or completely filled shell increases the electron-electron repulsion within that shell more sharply than any further increase in nuclear attraction can compensate for, so the outer electron cloud is pushed outward and the atom becomes measurably larger than its immediate neighbours to the left. Zinc, with the largest and most complete additional filling ($4s^2$ on top of a full $3d^{10}$), shows this effect most strongly, giving it the largest metallic radius of any first-row transition metal.

Putting the whole sequence together, the approximate order of increasing metallic radius is: $\text{Ni} < \text{Co} \approx \text{Fe} < \text{Mn} < \text{Cr} < \text{Cu} < \text{Zn}$.

Use the approximate radius values from the earlier section, order Cr through Zn, and explain the late rise using the same 'rising d-electron repulsion outweighs rising nuclear attraction' reasoning already developed for the plateau-then-rise trend.

Assuming radius decreases monotonically all the way from Cr to Zn (missing the late rise entirely); giving the rise no mechanistic explanation beyond 'it just increases'.