The Presence of Which of the Following Provides the Sample

Question

The presence of which of the following provides the sample with the lowest electron density?
A) Osmium
B) Lead
C) Uranium
D) Carbon
E) Gold

Accepted Answer

Contrast in the electron microscope depends on the electron-scattering power of the atoms in the specimen, which in turn depends on their atomic number. Thus all the atoms here have high atomic numbers, and high electron contrast, except carbon, which has an atomic number of 6 and is a major element in biological specimens to start with.