Fluorescein

A molecular ball-and-stick model of the fluorescein molecule, created using its crystal structure, Blender, and the molecular nodes plugin (by Brady Johnston). 

For printing this model in PLA with 0.4 mm nozzle, I recommend the following Cura slicer settings to make support removal easier: I used a 4 mm brim and tree supports (everywhere), support overhang 50 deg, 90% support interface density, 10 deg tree angle, and 40 mm base. Together this created a continuous support structure, minimizing the risk of detachment of parts. This also creates pretty well printed atoms/spheres. Of course, using a smaller nozzle improves the smoothness of the spheres. Also make sure that you place the print seam at one location, to prevent blobs all around the spheres.