{title2=$q=ze$}

An <ion> is a <chemical atom> or a group of <chemical atoms> with a nonzero net <electric charge>. A positive <ion> is a <cation>; a negative <ion> is an <anion>. The whole entity carries the <electric charge>: a <hydronium> <ion> is a group of four <chemical atoms> with net charge $+e$. This definition agrees with https://goldbook.iupac.org/terms/view/I03158[IUPAC Gold Book: ion]{ref}.

For an ordinary <ion> containing only <atomic nuclei> and <electrons>, let $N_p$ count all its <protons> and $N_e$ all its <electrons>. Each <proton> contributes the <elementary charge> $+e$, each <electron> contributes $-e$, and <neutrons> contribute zero. Therefore
$$
q=(N_p-N_e)e=ze,\qquad z=N_p-N_e.
$$
The signed integer $z$ is the net <electric charge> number. Removing an <electron> increases $z$ by one; adding an <electron> decreases $z$ by one. Changing the <electron> count leaves the <atomic number>, and therefore the identity of the <chemical element>, unchanged.

For example, a neutral <sodium> <chemical atom> has $11$ <protons> and $11$ <electrons>. Its <cation> $\mathrm{Na}^+$ still has $11$ <protons>, but only $10$ <electrons>, so $q=(11-10)e=+e$. A neutral <chlorine> <chemical atom> has $17$ <protons> and $17$ <electrons>; its <anion> $\mathrm{Cl}^-$ has $17$ <protons> and $18$ <electrons>, so $q=(17-18)e=-e$. These are particle inventories, not pictures of <atomic orbitals>. See <Chemistry 2e by Paul Flowers and others/2.6 Ionic and Molecular Compounds> (§2.6: ions and ionic compounds).

\Image[/ion-charge-bookkeeping.png]
{title=<Ion> <electric charge> bookkeeping: neutral <sodium> and <chlorine> <chemical atoms> compared with their <cation> and <anion>}
{description=Every colored dot counts one <proton> or one <electron>. Rows are counting arrays, not spatial models of <atomic nuclei> or <electron shells>. The net <electric charges> are $0,+e,0,-e$.}

One <sodium chloride> <formula unit> accounts for one $\mathrm{Na}^+$ <cation> and one $\mathrm{Cl}^-$ <anion>, whose net <electric charges> sum to zero. Ordinary solid <sodium chloride> is an extended arrangement of <ions>, rather than discrete $\mathrm{NaCl}$ <molecules>. This is a useful distinction: a neutral material can contain charged constituents, and its <chemical formula> tells us their ratio without claiming that each pair is a separate <molecule>.

<Ion> bookkeeping connects <chemistry> to <electric current>: moving <ions> can transport <electric charge>, while stationary <ions> cannot supply a transport <current density>. In <electrochemistry>, both signs of <ions> can contribute to the same <conventional current>. An <ion> opposing a fixed <electric charge> is called a <counterion>.


 Ion (source code)