Balancing equal charges

An ionic compound has no overall charge. Its formula shows the simplest whole-number ratio of its constituent ions, rather than a separate molecule.

One $Na^+$ balances one $Cl^-$: $+1+(-1)=0$, giving $NaCl$. One $Mg^{2+}$ balances one $O^{2-}$: $+2+(-2)=0$, giving $MgO$. Equal and opposite ion charges need a 1:1 ratio, regardless of their size.

Write the positive ion first. Use charges to work out the ratio, but do not include them in the neutral compound formula. Write $MgO$, not $Mg_2O_2$, because the ion ratio must be simplest.

Balancing different charges

If charges have different sizes, find the smallest common total charge.

For aluminium oxide, the matching charge size is 6: two aluminium ions give +6 and three oxide ions give −6. This is why the 2 belongs after Al and the 3 after O.

The cross-over shortcut gives the same answer if you simplify afterwards. For $Mg^{2+}$ and $O^{2-}$, crossing the charge numbers would first give 2:2, which must simplify to 1:1.

Keeping a group ion together

A group ion is treated as one unit when balancing charges. Do not change the formula inside it.

One $Ca^{2+}$ needs two $OH^-$ ions. Put the repeated group in brackets: $Ca(OH)_2$. The 2 applies to both O and H. $CaOH_2$ would have only one O for two H, so it fails to preserve two hydroxide groups.

Similarly, $Mg^{2+}$ with $NO_3^-$ gives $Mg(NO_3)_2$. One $Na^+$ with one $OH^-$ gives $NaOH$: brackets are unnecessary when only one group is needed. Two $Na^+$ with one $CO_3^{2-}$ give $Na_2CO_3$; keep the carbonate subscript 3 unchanged.

[DIAGRAM: asset_name: charge_balancing_ionic_formulae - diagram 1; asset_slug: 032_1_26_formulae_of_ionic_compounds_including_oxides_hydroxides_halides_nitrates_c_diagram1; file: 032_1_26_formulae_of_ionic_compounds_including_oxides_hydroxides_halides_nitrates_c_diagram1_imagegen_v1.png; recommended_method: deterministic_drawn; description: Exact assessed formula-deduction visual showing three monochrome worked charge balances: Mg2+ with two Cl- ions gives MgCl2; two Al3+ ions with three O2- ions gives Al2O3; Ca2+ with two OH- group ions gives Ca(OH)2. The visual should show total positive and negative charges adding to zero, use cation-first final formulae, and make the bracket rule for multiple hydroxide ions explicit. Deterministic drawing is used because formulae, charges, subscripts and brackets must be exact.]

The diagram compares a halide, an oxide and a hydroxide. The same zero-charge rule works for each; only a repeated group ion needs brackets.

Using the method across compound families

Use the supplied ions for oxides, hydroxides, halides, nitrates, carbonates and sulfates. A halide is a fluoride, chloride, bromide or iodide ion. The element chlorine is $Cl_2$, but a chloride ion is $Cl^-$; use the supplied ion formula.

For each answer, check: charges cancel; ratio is simplest; each group ion is unchanged; brackets enclose any repeated group; the final formula has no charge label.