How many hydrogen atoms are in a single molecule of water?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (B)

Reading chemical formulas is the key idea. In a water molecule, the formula is H2O, which shows two hydrogen atoms and one oxygen atom per molecule. The subscript after hydrogen (the little 2) tells you there are two hydrogen atoms bonded to the oxygen. So, there are two hydrogens in each water molecule. If you only had one hydrogen, you’d be describing a different substance; three or four hydrogens would point to other compounds and not to water. The two hydrogens share electrons with oxygen through covalent bonds, giving the characteristic arrangement of a water molecule.