The reason why twice as much hydrogen as oxygen is formed in the decomposition of water is due to the molecular composition of water. Water (H₂O) consists of two hydrogen atoms and one oxygen atom. When water is decomposed (for example, through electrolysis), it separates into its constituent elements: hydrogen gas (H₂) and oxygen gas (O₂).
The balanced chemical reaction for the decomposition of water is:
\[ 2 H₂O \rightarrow 2 H₂ + O₂ \]
This equation shows that for every two molecules of water (which contain four hydrogen atoms and two oxygen atoms), two molecules of hydrogen gas and one molecule of oxygen gas are produced. Hence, for every molecule of oxygen produced, there are two molecules of hydrogen produced, confirming that there is twice as much hydrogen as oxygen formed.
The correct explanation is therefore:
There are two atoms of hydrogen and one atom of oxygen in a water molecule.