= Stoichiometry of ionic compounds
{title2=$\sum_i\nu_i z_i=0$}
For a neutral <formula unit>, let $\nu_i$ count <ions> of type $i$ with <electric charge> $z_i e$, where $e$ is the <elementary charge>. The <stoichiometry> must satisfy
$$
\sum_i\nu_i z_i=0.
$$
For $\mathrm{Na}^+$ and $\mathrm{Cl}^-$, the smallest positive counts are $1:1$. For $\mathrm{Ca}^{2+}$ and $\mathrm{Cl}^-$, they are $1:2$, giving <calcium chloride> $\mathrm{CaCl_2}$: $1(2)+2(-1)=0$. The formula counts <ions>, not the number of nearest neighbours. A <counterion> is the oppositely charged partner needed to balance a charged species; its identity can change <solubility> and other properties without changing the identity of that species.<Chemistry 2e>[§7.1, neutral ionic formulas and charge balance; §2.6, ionic compound examples]{ref}
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