An atom is one neutral particle, a molecule is two or more atoms joined together, and an ion is a particle that carries a charge. Ask whether the particle is charged, then how many atoms it has.
This lesson opens the chapter on matter and atomic structure. Next you will count the particles inside an atom in calculating protons, neutrons and electrons.
What are the three definitions?
| Particle | What it is | Charge | Example |
|---|---|---|---|
| Atom | Smallest neutral particle of an element | None | Ne, Fe, C |
| Molecule | Two or more atoms joined by covalent bonds | None | O₂, H₂O, CO₂ |
| Ion | Atom or group of atoms that gained or lost electrons | Positive or negative | Na⁺, Cl⁻, SO₄²⁻ |
A molecule and an ion can both contain more than one atom. The difference is the charge, so check the charge first.
The two-question test
Question one: is the particle charged? If yes, it is an ion. If no, move to question two.
Question two: is it one atom or joined atoms? One atom means an atom. Joined atoms means a molecule.
Worked example: classify six substances
Apply the test to each substance.
| Substance | Charged? | Atoms joined | Answer | Usual wrong label |
|---|---|---|---|---|
| Neon gas, Ne | No | One | Atom | Molecule |
| Oxygen gas, O₂ | No | Two | Molecule of an element | Atom |
| Water, H₂O | No | Three | Molecule of a compound | Atom |
| Sodium chloride | Yes, Na⁺ and Cl⁻ | Lattice of ions | Ions | Molecule |
| Magnesium, Mg | No | One | Atom | Ion |
| Nitrate, NO₃⁻ | Yes | Four joined | Ion (polyatomic) | Molecule |
The last row shows the trap. NO₃⁻ has four atoms, but it is charged, so it is an ion and not a molecule.
What particles does each type of substance contain?
Covalent compounds such as water and carbon dioxide are made of molecules. Ionic compounds such as sodium chloride and magnesium oxide are made of ions. Metals are made of atoms in a sea of electrons.
If a question asks what particles a substance contains, the type of substance gives the answer. This also explains melting points, which you meet in explaining heating and cooling curves using particles.
Check yourself
Classify each as atom, molecule or ion: (a) argon, (b) CH₄, (c) Mg²⁺, (d) N₂.
Answer
(a) Argon is a single neutral particle, so it is an atom.
(b) CH₄ is neutral with five atoms joined, so it is a molecule of a compound.
(c) Mg²⁺ carries a charge, so it is an ion.
(d) N₂ is neutral with two atoms of the same element joined, so it is a molecule of an element.
What to study next
Continue with calculating protons, neutrons and electrons, where charge comes from counting electrons. Test the whole chapter with the practice set.
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