Formulas and equations
Formulas and equations
- An ionic compound is neutral — its positive and negative charges balance.
- You can predict many ion charges from the Periodic Table.
- Equations must be balanced, with state symbols.
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 $\text{O}^{2-}$).
Practice
A Group 2 element usually forms an ion with charge:
Group 2 elements lose two electrons to form 2+ ions; Group 17 form −1 ions.
Practice
Iron(III) oxide is Fe₂O₃ because:
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 (dissolved in water).
Practice
A balanced equation must have:
Atoms are conserved, so each element must appear in equal numbers on both sides.
Ionic equations
- An ionic equation shows only the ions and molecules that actually change.
- The unchanged ions are spectator ions — leave them out.
$$\text{Ag}^{+}(\text{aq}) + \text{Cl}^{-}(\text{aq}) \rightarrow \text{AgCl}(\text{s})$$
Practice
In an ionic equation, spectator ions are:
Spectator ions are unchanged by the reaction, so the ionic equation omits them.
You've got it
Key idea
- 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