Molecule vs Compound
Quick Answer
A molecule is any two or more atoms bonded together. A compound is a molecule made of different elements.
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What Each One Really Means
This one catches even senior students, because the definitions sound so similar. Let me settle it once and for all. A molecule is simply any group of atoms held together by chemical bonds. That is it. It can be the same element, it can be different elements. O2 is a molecule. H2 is a molecule. H2O is a molecule. But a compound is stricter. It must contain atoms of at least two different elements bonded together. So H2O is a compound and a molecule. But O2 is a molecule and not a compound. This means every compound is a molecule, but not every molecule is a compound. That sentence alone has failed many students in multiple choice exams. In my own teaching, I always tell them to ask two questions. First, are there chemical bonds? If yes, it is a molecule. Second, are there different elements? If yes, it is a compound. Two questions, two answers, no confusion. The tricky part is when students see something like NaCl and ask if it is a molecule. Well, in the solid state, NaCl is not really a molecule. It is a giant ionic lattice, a network of ions. But people still call it a compound, because it has different elements bonded in a fixed ratio. The molecule language is most useful for covalent substances. For ionic substances, we usually just say compound.
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Side-by-Side Comparison
| Aspect | Molecule | Compound |
|---|---|---|
| Minimum atoms | Two | Two, and they must be different elements |
| Can be one element? | Yes, e.g. O2 | No, must be different elements |
| Example | O2, H2, H2O | H2O, CO2, NaCl |
| Relationship | Broader category | Narrower category inside molecules |
| Bond type | Usually covalent | Covalent or ionic |
Memory Device
All compounds are molecules. Not all molecules are compounds. Say it until it sticks.