  The spatial distribution of mono-abundance populations (MAPs, selected in
[Fe/H] and [Mg/Fe]) reflect the chemical and structural evolution in a galaxy
and impose strong constraints on galaxy formation models. In this paper, we use
APOGEE data to derive the intrinsic density distribution of MAPs in the Milky
Way, after carefully considering the survey selection function. We find that a
single exponential profile is not a sufficient description of the Milky Way's
disc. Both the individual MAPs and the integrated disc exhibit a broken radial
density distribution; densities are relatively constant with radius in the
inner Galaxy and rapidly decrease beyond the break radius. We fit the intrinsic
density distribution as a function of radius and vertical height with a 2D
density model that considers both a broken radial profile and radial variation
of scale height (i.e., flaring). There is a large variety of structural
parameters between different MAPs, indicative of strong structure evolution of
the Milky Way. One surprising result is that high-$\alpha$ MAPs show the
strongest flaring. The young, solar-abundance MAPs present the shortest scale
height and least flaring, suggesting recent and ongoing star formation confined
to the disc plane. Finally we derive the intrinsic density distribution and
corresponding structural parameters of the chemically defined thin and thick
discs. The chemical thick and thin discs have local surface mass densities of
5.62$\pm$0.08 and 15.69$\pm$0.32 ${\rm M_{\odot} pc^{-2}}$, respectively,
suggesting a massive thick disc with a local surface mass density ratio between
thick to thin disc of 36%.
