Question

Fructose and galactose both have six carbon atoms, twelve hydrogen atoms and six oxygen atoms, just like glucose. a friend of yours says: "since those sugars have the same number of atoms, they should have the same name." you answer: "while they do have the same number and type of atoms

Answer 1

Answer is: they are named differently because they are alternate forms (isomers) of the monosaccharide, glucose.

An isomer of a molecule has the same number of atoms of each element (in this example, glucose, galactose and fructose have molecular formula C₆H₁₂O₆), but has a different arrangement of the atoms.

Difference between D-glucose and D-galactose  is the orientation of the hydroxyl group (OH) at fourth carbon atom, at D-glucose -OH group is right and at D-galactose is left.

Difference between D-fructose and D-glucose is that first one is ketonic monosaccharide and second one is aldose (carbonyl group on the endmost carbon atom).