Glucose and fructose have the same molecular formula, C6H12O6, but different structures. They are therefore

Explanation

Isomers share a molecular formula but differ in the arrangement of their atoms, and here glucose carries an aldehyde group while fructose carries a ketone group, which is why only glucose forms a six membered ring. The difference in structure gives a different taste and a different metabolic route despite the identical formula. Isotopes are forms of one element differing in neutron number.