Final answer:
The chemical group that is unique for each of the 20 amino acids is called the R group (or side chain). It determines the chemical nature and properties of each amino acid and contributes to the wide range of functions and structures of proteins.
Step-by-step explanation:
The chemical group that is unique for each of the 20 amino acids is called the R group (or side chain). In proteins, amino acids are linked together by peptide bonds, and each amino acid contributes its own unique R group, which determines its chemical nature and properties. For example, some amino acids have polar R groups, some have nonpolar R groups, and others have charged R groups. This diversity in R groups allows for the wide range of functions and structures of proteins.