Quote:

The size and shape of a molecule depend on the type and number of atoms that make up the molecule and how they are arranged. The smallest molecules?such as hydrogen, oxygen, and water molecules?contain only a few atoms. These molecules are smaller than one-millionth of a meter at their widest point. Scientists usually measure them in Angstroms (?), where one ? is 10-10 (or 1/10,000,000,000) meters. The hydrogen molecule, made of two hydrogen atoms, is about 1.5 ?. The oxygen molecule, made of two oxygen atoms, is slightly larger, since oxygen atoms are slightly larger than hydrogen atoms are.

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my conclusion from this text is that O2 is smaller that H2O because O2 is slightly bigger than H2 and H2O contains an extra O atom.

and ofcoarse CO2 is bigger than O2