Science 10 · Compounds and equations
Names, formulas and balancing
Two atoms can either share electrons or hand them over, and which one happens decides nearly everything about the substance that results. Then there is a naming system, and one law that makes balancing necessary.
- 1. The words, first
- 2. Two ways to make a compound
- 3. The naming system, as a decision tree
- 4. Balancing, and the law behind it
- 5. What costs marks
The words, first
The idea: Ionic and molecular are the two families, and every naming rule depends on telling them apart first.
| Word | What it means |
|---|---|
| Ionic compound | Metal plus non-metal. Electrons transfer, ions form, and they arrange into a lattice — not molecules. |
| Molecular compound | Non-metal plus non-metal. Electrons are shared in covalent bonds, forming discrete molecules. |
| Formula unit | The simplest whole-number ratio of ions in an ionic compound. NaCl, never Na₂Cl₂. |
| Polyatomic ion | A charged group that stays together: NO₃⁻, SO₄²⁻, CO₃²⁻, OH⁻, NH₄⁺. |
| Multivalent metal | Forms more than one ion, so the name needs a Roman numeral: iron(II) and iron(III). |
| Prefix | mono-, di-, tri-, tetr-, pent-. Used for molecular compounds only. |
| Reactant / product | What goes in; what comes out. Left and right of the arrow. |
| Coefficient | The big number in front of a formula in an equation. The only thing you may change when balancing. |
| Subscript | The small number inside a formula. Changing it changes the substance, so it is never touched when balancing. |
| Law of conservation of mass | Atoms are rearranged, not created or destroyed, so total mass is unchanged. |
Two ways to make a compound
The idea: The difference in behaviour is large, and it all comes from whether electrons were shared or handed over.
| Ionic | Molecular | |
|---|---|---|
| Made from | Metal + non-metal | Non-metal + non-metal |
| Electrons | Transferred | Shared |
| Structure | A lattice of ions | Separate molecules |
| Melting point | High — the whole lattice must break | Low — only molecules must separate |
| Dissolved in water | Conducts, because ions are free to move | Usually does not |
Why salt water conducts and sugar water does not. NaCl separates into Na⁺ and Cl⁻, and moving charges are a current. Sugar dissolves as whole neutral molecules, so nothing is carrying charge.
Why methane melts at −182 °C and salt at 801 °C. Not because methane's bonds are weak — they are strong — but because melting it only has to pull whole molecules apart from each other.
The naming system, as a decision tree
The idea: Four questions in order, and each one halves what is left.
- Is the first element a metal? No → molecular, use prefixes, done. Yes → ionic, continue.
- Is that metal multivalent? Yes → work out its charge from the anion and write a Roman numeral.
- Is the anion polyatomic? Yes → use its own name (nitrate, sulfate). No → element name with -ide.
- Balance the charges for the subscripts, bracketing any polyatomic ion needed more than once.
Worked. MgCl₂: magnesium is a metal, not multivalent, chloride is simple. Magnesium chloride — and no prefixes, because the charges already fix the ratio.
Worked. FeCl₂: iron is multivalent. Two chlorides carry 2−, so iron is 2+. Iron(II) chloride. The numeral names the charge, not the number of chlorines.
Worked. N₂O₅: two non-metals, so prefixes on both. Dinitrogen pentoxide — the a of penta- drops before the vowel.
Worked. Calcium nitrate: Ca²⁺ with NO₃⁻ needs two nitrates, and a repeated polyatomic ion takes brackets: Ca(NO₃)₂.
Balancing, and the law behind it
The idea: The same atoms come out as went in. Balancing is bookkeeping for that fact, not a puzzle for its own sake.
The rule. Change only the coefficients. Changing a subscript changes the substance — H₂O and H₂O₂ are water and hydrogen peroxide, and swapping one for the other is not balancing.
Worked: hydrogen burning. H₂ + O₂ → H₂O. Oxygen is unbalanced, so try 2 in front of the water: H₂ + O₂ → 2H₂O. Now hydrogen is 2 on the left and 4 on the right, so put 2 in front of H₂:
2H₂ + O₂ → 2H₂O (4 H and 2 O on each side ✓)
Worked: methane burning. Balance carbon and hydrogen first, and leave oxygen to last, because it appears in two products:
CH₄ + 2O₂ → CO₂ + 2H₂O
Why mass seemed not to be conserved for so long. Burn something in the open and it appears to lose mass, because a gas leaves. Do it in a sealed container and the balance does not move. Lavoisier's sealed vessels are what settled it.
What costs marks
The idea: Two naming, two balancing.
- Prefixes on an ionic compound. CaCl₂ is calcium chloride, never calcium dichloride.
- Missing brackets around a repeated polyatomic ion.
- Changing a subscript to balance. Only coefficients may change.
- Counting atoms without multiplying through the coefficient. 2H₂O is 4 hydrogens and 2 oxygens.