Choose the Best-Fit R-Group. The alpha carbons are colored green.
Guess before you reveal the sidechain by consulting a table of amino acids ( found here).

Gly or Ala or Ser   
Leu or Arg or Lys   
Gly or Ala or Ser   
Arg or Lys   
Tyr or Phe or His or Trp   
Tyr or Phe or His or Trp   
Gly or Ala or Ser   
Val or Leu or Ile   
A short peptide in its Electron Density Map.
The Jmol image shows a short polypeptide within its electron density map. The segment of a protein structure is initially shown as an Gly peptide displayed as Backbone. The meshwork surrounding the chain simulates the electron density contour from an X-Ray diffraction experiment.

Guess the most appropriate amino acid from the suggestions and then use the button to reveal the correct sidechain.