The group that is not attached to the central carbon atom (alpha carbon) in an amino acid is an oxygen atom alone. In a typical amino acid structure, the central carbon atom is attached to:

• An amino functional group (-NH2)
• A carboxyl functional group (-COOH)
• A hydrogen atom
• A side chain (R group), which varies among different amino acids

Oxygen is present in the carboxyl group but not directly attached as a single oxygen atom to the central carbon. Therefore, an oxygen atom by itself is not attached to the central carbon atom in an amino acid.