Well Al2O3 has 2 atoms of aluminum (Al2) and 3 atoms of Oxygen (O3). Thus, as there are 2Al2O3 molecules there are 4 atoms of aluminum.
Molecules and elements are not the same thing. There are many molecules in elements but not elements inside of molecules.
There are many different molecules. You should specify a specific molecule
How many elements are in 2mgso4
4Al + 3 O2 -------> 2Al2O3 so 4.20 x 3/4 = 3.15 moles of oxygen molecules
They are many as the oxides of many elements.
Diatomic molecules are molecules composed of two different elements. Examples include H2 , O2 , N2 , and many other elements in the same structure.
They are both elements, form molecules, and will bond with many other elements.