For d block elements the first ionization potential is of the order

The ionisation energies increase with increase in atomic number. However, the trend is some irregular among d-block elements. On the basis of electronic configuration, the
 

$\mathrm{Zn}:1{\mathrm{s}}^{2}2{\mathrm{s}}^2{\mathrm{p}}^{6}3{\mathrm{s}}^2{\mathrm{p}}^6{\mathrm{d}}^{10}4{\mathrm{s}}^2$
$\mathrm{Fe}:1{\mathrm{s}}^{2}2{\mathrm{s}}^2{\mathrm{p}}^{6}3{\mathrm{s}}^2{\mathrm{p}}^6{\mathrm{d}}^{6}4{\mathrm{s}}^2$
$\mathrm{Cu}:1{\mathrm{s}}^{2}2{\mathrm{s}}^2{\mathrm{p}}^{6}3{\mathrm{s}}^2{\mathrm{p}}^6{\mathrm{d}}^{10}4{\mathrm{s}}^1$
$\text{ Cr: }1{\mathrm{s}}^{2}2{\mathrm{s}}^2{\mathrm{p}}^{6}3{\mathrm{s}}^2{\mathrm{p}}^6{\mathrm{d}}^{5}4{\mathrm{s}}^1$
${\mathrm{IE}}_1\text{ follows the order : }\mathrm{Zn}>\mathrm{Fe}>\mathrm{Cu}>\mathrm{Cr}$