Cation vs anion
Core definitions
Cation: an ion with a net positive charge, produced when an atom (or group) has fewer electrons than protons.
Anion: an ion with a net negative charge, produced when an atom (or group) has more electrons than protons.
Charge and electron accounting
Ion charge reflects the difference between proton count and electron count. Protons remain in the nucleus during ordinary chemical change, while electrons are transferred, shared, or redistributed.
- Positive charge: electron deficit relative to the neutral atom (typical of metals forming cations).
- Negative charge: electron excess relative to the neutral atom (typical of nonmetals forming anions).
- Magnitude of charge: the integer difference in electrons compared with the neutral species (for monatomic ions in introductory chemistry).
Representative half-reactions (electron transfer language):
Comparative summary
| Feature | Cation | Anion |
|---|---|---|
| Net charge sign | \(+\) | \(-\) |
| Electron change (typical) | Electron loss relative to the neutral atom | Electron gain relative to the neutral atom |
| Periodic-table tendency | Metals (especially Groups 1 and 2) form cations readily | Nonmetals (especially Groups 16 and 17) form anions readily |
| Ionic radius vs neutral atom | Usually smaller (reduced electron–electron repulsion; sometimes loss of a valence shell) | Usually larger (increased electron–electron repulsion in the valence shell) |
| Electrolysis migration language | Attracted toward the cathode (negative electrode) | Attracted toward the anode (positive electrode) |
| Examples | \(\mathrm{Na}^{+},\ \mathrm{Ca}^{2+},\ \mathrm{Al}^{3+},\ \mathrm{NH_4}^{+}\) | \(\mathrm{Cl}^{-},\ \mathrm{O}^{2-},\ \mathrm{NO_3}^{-},\ \mathrm{SO_4}^{2-}\) |
Aqueous solutions and electrolytes
In water, many ionic compounds separate into cations and anions. Strong electrolytes dissociate essentially completely, producing mobile ions that carry electric current.
Charge balance in solution follows from conservation of charge. For dissolved \(\mathrm{CaCl_2}\), the total positive charge (\(+2\)) matches the total negative charge (\(2\times -1\)).
Visualization: electron transfer, charge sign, and size trend
Electron configuration viewpoint
Monatomic ions often reach a noble-gas electron count. The sodium cation \(\mathrm{Na}^{+}\) has 10 electrons (neon-like), and the chloride anion \(\mathrm{Cl}^{-}\) has 18 electrons (argon-like). This electron-count framing aligns with the stability of filled valence shells introduced in general chemistry.
Common pitfalls
- Charge sign vs element symbol: the sign belongs to the ion, not to the element name; \(\mathrm{Cl}\) and \(\mathrm{Cl}^{-}\) are distinct species.
- Subscripts vs charges: subscripts count atoms in a formula unit (for example, \(\mathrm{CaCl_2}\)); superscripts show ionic charge (for example, \(\mathrm{Ca}^{2+}\)).
- Polyatomic ions: the entire group carries the charge (for example, \(\mathrm{SO_4}^{2-}\) is one anion, not separate charged atoms).
A cation is a positively charged ion produced by electron loss, and an anion is a negatively charged ion produced by electron gain.