While hydrogen and oxygen are both relatively light elements, the weight of water primarily stems from the combination of these elements in a molecule.
Water (H2O) is formed when two hydrogen atoms (H) chemically bond with one oxygen atom (O). The molecular structure of water consists of two hydrogen atoms covalently bonded to an oxygen atom. Although hydrogen and oxygen individually have low atomic masses, the combination of these atoms into a water molecule results in a more substantial mass due to the overall structure.
The atomic mass of hydrogen is approximately 1 atomic mass unit (AMU), and oxygen has an atomic mass of around 16 AMU. In a water molecule, two hydrogen atoms combine with one oxygen atom. Therefore, the molecular mass of water (H2O) is calculated as the sum of the atomic masses of its constituent elements: 2(1 AMU) + 16 AMU = 18 AMU.
So, despite the lightness of its constituent elements, the water molecule itself has a relatively higher molecular weight due to the combination of these atoms.