  A series of f-block chromates, CsM(CrO$_4$)$_2$ (M = La, Pr, Nd, Sm, Eu; Am),
were prepared revealing notable differences between the AmIII derivatives and
their lanthanide analogs. While all compounds form similar layered structures,
the americium compound exhibits polymorphism and adopts both a structure
isomorphous with the early lanthanides as well as one that possesses lower
symmetry. Both polymorphs are dark red and possess band gaps that are smaller
than the LnIII compounds. In order to probe the origin of these differences,
the electronic structure of $\alpha$-CsSm(CrO$_4$)$_2$,
$\alpha$-CsEu(CrO$_4$)$_2$ and $\alpha$-CsAm(CrO$_4$)$_2$ were studied using
both a molecular cluster approach featuring hybrid density functional theory
and QTAIM analysis, and by the periodic LDA+GA and LDA+DMFT methods. Notably,
the covalent contributions to bonding by the f orbitals was found to be more
than twice as large in the AmIII chromate than in the SmIII and EuIII
compounds, and even larger in magnitude than the Am-5f spin-orbit splitting in
this system. Our analysis indicates also that the Am-O covalency in
$\alpha$-CsAm(CrO$_4$)$_2$ is driven by the degeneracy of the 5f and 2p
orbitals, and not by orbital overlap.
