Atoms & molecules
The idea
Zoom in far enough on anything — a coin, a drop of juice, the air in a balloon — and you reach atoms, the basic building units of matter. You already know matter is made of particles too small to see; atoms are what most of those particles are, or are built from. When two or more atoms bond together into a fixed group, that group is a molecule. A chemical formula is the molecule's recipe: H₂O says each water molecule is exactly two hydrogen atoms bonded to one oxygen atom.
Read a formula like a parts list: the small subscript counts the atoms of the element written just before it, and no subscript means one. So CO₂ holds one carbon and two oxygens. A common trap is imagining an atom of water. There is no such thing — water is a molecule, and the smallest possible piece of water is one whole H₂O. Split that molecule apart and you no longer have water at all; you have hydrogen and oxygen, two completely different substances.
Worked example
The sugar your body burns for energy is glucose, with the formula C₆H₁₂O₆. How many atoms of each element are in one glucose molecule, and how many atoms in total are in 4 glucose molecules?
- Read the subscripts as counts: C₆ means 6 carbon atoms, H₁₂ means 12 hydrogen atoms, and O₆ means 6 oxygen atoms in every single molecule.
- Add the counts to get the size of one molecule: 6 + 12 + 6 = 24 atoms per glucose molecule.
- Four identical molecules carry four times as many atoms: 4 × 24 = 96 atoms in total.
- Sanity-check by counting element by element: 4 × 6 = 24 carbons, 4 × 12 = 48 hydrogens, 4 × 6 = 24 oxygens, and 24 + 48 + 24 = 96 — the two routes agree.
Answer. One glucose molecule contains 6 carbon, 12 hydrogen, and 6 oxygen atoms (24 in all), so 4 molecules contain 96 atoms.
Check your understanding
- Why is the smallest possible piece of water a molecule rather than an atom, and what would you actually get if you split that molecule?
- How would you explain the difference between an atom and a molecule to a friend using building blocks or beads?
- What changes about a substance if every one of its molecules swaps a single atom for a different kind of atom?
- How does the formula CO₂ tell a different story from the formula CO, even though both contain the same two elements?
Build the foundations first
Atoms & molecules builds on these concepts. If any feel shaky, start there.