The number of Oxygen atoms in both of glucose and fructose is 6 atoms, which means 12 in total ,but the sucrose is 11 atoms of Oxygen although it's composed of fructose and glucose, the question where the atom number 12 go ?

The atom number 12 of oxygen contributes to the formation of water (H2O) during the process of forming sucrose.

Explanation:

Sucrose is a dissacharide carbohydrate i.e it contains two simple sugars or saccharides. The two simple sugar monomeric components of sucrose are glucose and fructose. As rightly stated in this question, the number of Oxygen atoms in both of glucose and fructose is 6 atoms respectively, which means 12 in total, however, the sucrose molecule has 11 atoms of oxygen meaning 1 oxygen atom is missing.

Sucrose is formed from glucose and fructose by a process called DEHYDRATION REACTION. A dehydration reaction means that one MOLECULE OF WATER IS LOST in order to form a glycosidic linkage/bond that joins fructose and glucose monomeric sugars together in the sucrose molecule. Two hydrogen atoms and ONE OXYGEN atom from the supposed 12 is used to form the WATER MOLECULE.