Fructose and glucose are

Step 1: Analyze the question The question asks how fructose and glucose are related. Each option uses a different chemistry term, so we need to know what each one means.

Step 2: Key concepts

• Allotropes: different forms of the same element (e.g., diamond and graphite are both carbon).
• Disaccharides: sugars made of two monosaccharide units (e.g., sucrose, maltose, lactose).
• Dimers: molecules formed by joining two identical smaller units.
• Isomers: compounds with the same molecular formula but different structural arrangements.
• Isotopes: atoms of the same element with the same number of protons but different numbers of neutrons.

Step 3: Apply to glucose and fructose

• Both have the molecular formula C₆H₁₂O₆.
• Both are monosaccharides, so B is wrong.
• Glucose is an aldohexose (it has an aldehyde group, -CHO).
• Fructose is a ketohexose (it has a ketone group, C=O).

Since they share a molecular formula but have different structures, they are structural isomers (more precisely, functional group isomers).

Step 4: Eliminate the wrong options

• A: Allotropy applies only to elements, not compounds. ✗
• B: Both are monosaccharides, not disaccharides. ✗
• C: They are not formed by joining two of each other. (Joined together, they make sucrose, a different compound.) ✗
• E: Isotopes apply to atoms, not molecules. ✗

Answer: D. isomers of each other

Exam tip: Remember "Glucose has an Aldehyde, Fructose has a Ketone". Whenever you see two compounds with the same molecular formula, think isomers. Also, allotropes and isotopes are about elements/atoms, not compounds like sugars.