Formulas and equations
Introduced| English | Meaning |
|---|---|
| ionic compound/aɪˈɒnɪk ˈkɒmpaʊnd/ | A substance made of positive and negative ions held together by electrostatic attraction. |
| balanced/ˈbælənst/ | Describing a chemical equation with the same number of atoms of each element on both sides and consistent total charge. |
| state symbol/steɪt ˈsɪmbl/ | A label in a chemical equation identifying a substance as solid, liquid, gas or dissolved in water. |
| aqueous/ˈeɪkwɪəs/ | Dissolved in water, or relating to a solution whose solvent is water. |
| ionic equation/aɪˈɒnɪk ɪˈkweɪʒn/ | An equation showing the ions and other species that take part in a reaction, omitting spectator ions. |
| spectator ion/spekˈteɪtə ˈaɪɒn/ | Ions present in a reaction mixture that undergo no net chemical change. |
The language of reactions
- An ionic compound 离子化合物 is neutral — its positive and negative charges balance.
- You can predict many ion charges from the Periodic Table.
- Equations must be balanced 配平, with state symbols 状态符号.
A balanced equation has the same number of each type of atom on both sides.
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Atoms are neither created nor destroyed, so mass is conserved.
Ion charges
| Group | 1 | 2 | 13 | 15 | 16 | 17 |
|---|---|---|---|---|---|---|
| Ion charge | +1 | +2 | +3 | −3 | −2 | −1 |
- Some to learn: nitrate $\text{NO}_3^{-}$, carbonate $\text{CO}_3^{2-}$, sulfate $\text{SO}_4^{2-}$, hydroxide $\text{OH}^{-}$, ammonium $\text{NH}_4^{+}$.
- Balance charges to get the formula: iron(III) oxide is $\text{Fe}_2\text{O}_3$ (two $\text{Fe}^{3+}$ balance three $\text{O}^{2-}$).
A Group 2 element usually forms an ion with charge:
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Group 2 elements lose two electrons to form 2+ ions; Group 17 form −1 ions.
Match each group to the ion charge it usually forms.
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Atoms lose or gain electrons to reach a full shell; the charge tells you how to combine them into a neutral formula.
Iron(III) oxide is Fe₂O₃ because:
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Charges must balance: 2 × (+3) = +6 and 3 × (−2) = −6, giving Fe₂O₃.
Balancing and state symbols
- A balanced equation has the same number of each atom on both sides.
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous 水溶液 (dissolved in water).
Equation balancing route
Follow atoms through a chemical equation so both sides match.
A balanced equation must have:
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Atoms are conserved, so each element must appear in equal numbers on both sides.
Balance this equation: ___ H₂ + O₂ → 2 H₂O. What number goes in front of H₂?
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2 H₂ + O₂ → 2 H₂O: 4 H and 2 O on each side. The coefficient of H₂ is 2.
Ionic equations 离子方程式
- An ionic equation shows only the ions and molecules that actually change.
- The unchanged ions are spectator ions 旁观离子 — leave them out.
$$\text{Ag}^{+}(\text{aq}) + \text{Cl}^{-}(\text{aq}) \rightarrow \text{AgCl}(\text{s})$$
In an ionic equation, spectator ions are:
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Spectator ions are unchanged by the reaction, so the ionic equation omits them.
You've got it
- ionic compounds are neutral: balance + and − charges (e.g. $\text{Fe}_2\text{O}_3$)
- group ion charges: 1→+1, 2→+2, 13→+3, 15→−3, 16→−2, 17→−1
- balanced equation = same atoms both sides; state symbols (s)(l)(g)(aq)
- ionic equations show only what changes; spectator ions are omitted