Atomization enthalpies of transition elements So, my book says that transition elements have higher enthalpies of atomization than other elements (say s- or p- block) because of stronger metallic bonding, primarily due to large number of unpaired electrons in the d-orbitals. 

Now, if that is the case, then the atomization energy for $\ce{Ni}$ should be less than for $\ce{Fe}$ and $\ce{Co}$, as $\ce{Ni}$ has only 2 unpaired electrons compared to $\ce{Fe}$ and $\ce{Co}$ (4 and 3 respectively).  

Actually, it is just the opposite.

$$\begin{array}{cc}
\text{Metal} &amp; \Delta_\text{atom}H(\ce{M})\text{ / }\mathrm{kJ~mol^{-1}} \\ \hline
\ce{Ni} &amp; 430 \\
\ce{Co} &amp; 425 \\
\ce{Fe} &amp; 416
\end{array}$$

Why could this be?
The sorts of generalities always sweep aside other considerations.

$\ce{^26Fe}$ has a ​body-centered cubic crystal structure. 

$\ce{^27Co}$ has a ​hexagonal close-packed crystal structure. 

$\ce{^28Ni}$ has a ​face-centered cubic crystal structure. 

Here is a nice table showing the overall trends.