Biological material is mainly composed of molecules containing atoms like
carbon, hydrogen, oxygen, nitrogen, phosphor etc., which all have a low atomic
weight and are therefore electron-transparent. In the electron microscope such
untreated samples would yield a blurred image with low contrast. To increase
contrast, samples have to be treated with salts of heavy metals which are able
to scatter electrons.
See section Stains in Electron Microscopy for the most common contrasting reagents.