Filled statement
Metals gain a positive charge and become a cation.
Charge meaning in terms of protons and electrons
The net charge of any atom or ion depends on how many protons and electrons it contains. Protons carry \(+1\) charge each and electrons carry \(-1\) charge each, so the net charge is
\[ \text{net charge} = (\#\text{protons}) - (\#\text{electrons}) \]
Neutral atoms have equal numbers of protons and electrons, giving a net charge of \(0\). A positive ion has fewer electrons than protons, giving a net charge \(>0\).
Why metals become positive ions
Metals typically have low ionization energy compared with nonmetals in the same period, so losing valence electrons is energetically favorable relative to gaining electrons. Loss of electrons reduces the electron count while the proton count stays fixed, producing a positive charge.
Electron loss is oxidation. When a metal atom forms a cation, the metal has been oxidized.
Representative half-reactions for common metal cations
The cation charge equals the number of electrons lost. Typical examples are summarized by oxidation half-reactions:
| Metal atom | Electrons lost | Cation formed | Half-reaction (electron loss) |
|---|---|---|---|
| \(\mathrm{Na}\) | 1 | \(\mathrm{Na^+}\) | \(\mathrm{Na \rightarrow Na^+ + e^-}\) |
| \(\mathrm{Mg}\) | 2 | \(\mathrm{Mg^{2+}}\) | \(\mathrm{Mg \rightarrow Mg^{2+} + 2e^-}\) |
| \(\mathrm{Al}\) | 3 | \(\mathrm{Al^{3+}}\) | \(\mathrm{Al \rightarrow Al^{3+} + 3e^-}\) |
| \(\mathrm{Ca}\) | 2 | \(\mathrm{Ca^{2+}}\) | \(\mathrm{Ca \rightarrow Ca^{2+} + 2e^-}\) |
Connection to electron configurations
Cations have fewer electrons than the neutral atoms. Main-group metals tend to lose their outer (valence) electrons to reach a more stable noble-gas-like configuration. For a typical metal in Group 1, one valence electron is lost; for a typical metal in Group 2, two valence electrons are lost.
Visualization of electron loss producing a cation
Common pitfalls
“Gain a positive charge” reflects the final net charge, not the motion of positive particles. The physical process in metals is electron loss; protons remain in the nucleus and do not transfer during ordinary chemical reactions.
A cation is any positively charged ion, metal or nonmetal. Metals form cations most commonly because electron loss from metals is favored by periodic trends in ionization energy and metallic character.