Ionization
Definition and meaning of Ionization in chemistry.
Ionization is the process by which a neutral atom, molecule, or compound gains or loses one or more electrons to become an electrically charged ion.
In more detail
Losing an electron produces a positively charged cation, while gaining an electron produces a negatively charged anion. Ionization can be driven by energy input, such as heat, light (photoionization), or collision with another particle, or it can occur when a molecular (covalent) compound reacts with a polar solvent like water and forms new ions that did not exist in the pure substance, for example, hydrogen chloride gas ionizing in water to form H⁺ and Cl⁻. This is distinct from an already-ionic solid such as NaCl simply separating into its pre-existing ions on dissolution, which is dissociation rather than ionization. The energy required to remove the most loosely held electron from a gaseous atom is called its first ionization energy, and this quantity generally increases across a period and decreases down a group of the periodic table because of changes in nuclear charge and atomic radius.
Key facts
| Field | General Chemistry |
|---|---|
| Typical unit | kJ/mol (ionization energy) |
| Result | Cation (loses e⁻) or anion (gains e⁻) |
| Example value | H: 1312 kJ/mol (1st ionization energy) |
A sodium atom (Na) loses one electron to form a sodium cation and a free electron: Na → Na⁺ + e⁻. This requires 496 kJ/mol, sodium's first ionization energy.
Frequently asked questions
Is ionization the same as dissociation?
No. Dissociation is the separation of an already-ionic compound into its existing ions when it dissolves (e.g., NaCl into Na⁺ and Cl⁻), while ionization is the actual creation of a charge by adding or removing electrons from a neutral species.
Why does ionization energy generally increase across a period?
Across a period, nuclear charge increases while atomic radius shrinks, so valence electrons are held more tightly and require more energy to remove.