Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- A particle diagram shows a balanced equation as atoms, molecules and ions. You must be able to draw one from an equation and read one back into an equation.
- A diagram needs a key. Use letters or labels as well as shading.
- A chemical change rearranges atoms into new combinations. A physical change keeps the same particles and changes only their spacing or arrangement.
Key relationships
| Rule | What to check |
|---|---|
| Atoms conserved | Count each element before and after; totals match |
| Charge conserved | In any solution, total positive charge = total negative charge |
| Reacting ratio | Particles used and formed are in the coefficient ratio (the box can hold any multiple) |
| Excess reactant | Leftover particles appear unchanged after; they are not in the equation |
| Solutions | Soluble ionic compound → separate ions in formula ratio; polyatomic ions stay whole; molecular solutes stay whole |
| Precipitate | Packed cluster of ions, usually at the bottom |
| States | Solid: packed, regular. Liquid: close, disordered. Gas: far apart |
Reading a diagram into an equation: count particles → count atoms (check) → subtract leftovers → reduce the used-and-formed numbers to the smallest whole-number ratio → add state symbols.
Assumptions behind the pictures
- Water molecules in a solution are usually left out for clarity; say so in a label.
- A diagram shows a tiny sample. Only the ratio of particles matters, not the actual number.
- Sizes and distances are not to scale.
Mistakes to avoid
- Thinking the number of molecules must stay the same.
- Leaving excess reactant out of the “after” box, or putting it into the equation.
- Drawing NaCl(aq) as NaCl pairs instead of separate ions.
- Splitting SO₄²⁻ or NO₃⁻ into separate atoms.
- Getting the ion ratio wrong: K₂SO₄(aq) has two K⁺ for every SO₄²⁻.
- Drawing boiling water as H₂ and O₂.
Quick self-check
- Is 2 C₂H₆ + 7 O₂ → 4 CO₂ + 6 H₂O shown correctly by a box with 4 C, 12 H and 14 O atoms on each side? (Yes: C 4, H 12, O 14 before and after.)
- How should two formula units of Al₂(SO₄)₃(aq) be drawn? (4 separate Al³⁺ ions and 6 separate SO₄²⁻ ions; charge 4(+3) + 6(−2) = 0.)
- A box shows 5 H₂O(l) before and 5 H₂O(g) after. Physical or chemical? (Physical: the same molecules, only farther apart.)
Next: practice questions.