Final answer:
In the chemical formula H₂O, the subscript '2' means there are two hydrogen atoms in one molecule of water.
Step-by-step explanation:
In the chemical formula H₂O, the subscript '2' represents the number of hydrogen atoms in one molecule of water. Therefore, the correct answer is: a) Number of hydrogen atoms. This subscript indicates that there are two hydrogen atoms bonded to one oxygen atom within a single molecule of water. In contrast, if there were a coefficient before the formula, such as 2H₂O, this would indicate two separate water molecules, each still with two hydrogen and one oxygen atom.
It's crucial to differentiate between subscripts and coefficients in chemical formulas. A subscript applies to the atom it's attached to and tells us the number of those atoms in one molecule, whereas a coefficient applies to the entire formula and indicates how many of those molecules there are. Therefore, we cannot simply change the subscripts in a molecular formula, as this would represent a different compound. For example, changing the formula of water (H₂O) to include a subscript of 2 on the oxygen (making it H₂O₂) would change the compound from water to hydrogen peroxide, which has very different properties.