* Water is a compound, oxygen and nitrogen are elements. Water is made up of two hydrogen atoms and one oxygen atom. Oxygen and nitrogen, on the other hand, exist as diatomic molecules, meaning they consist of two atoms of the same element bonded together.
* Weight comparison: You're comparing a molecule of water with molecules of oxygen and nitrogen. You should be comparing the weight of one oxygen atom to one hydrogen atom, and one nitrogen atom to one hydrogen atom.
* Individual atomic weights: Oxygen is heavier than hydrogen, and nitrogen is heavier than hydrogen.
In summary: Water molecules are not lighter than oxygen and nitrogen molecules. Instead, water is a compound composed of lighter hydrogen atoms and heavier oxygen atoms. This makes a single water molecule lighter than a single oxygen or nitrogen molecule, but the individual atoms that make up water are not lighter than the individual atoms of oxygen or nitrogen.
