The monosaccharides glucose, galactose, and fructose share the identical molecular formula $C_6H_{12}O_6$ but have different bonding sequences. Glucose and galactose have a six-carbon chain with a double-bonded oxygen on the first carbon, differing in the arrangement of their hydroxyl ($-OH$) groups. Fructose has its double-bonded oxygen on the second carbon of the chain.