Ionization
Definition and meaning of Ionization in chemistry.
Ionization is the process by which a neutral atom or molecule gains or loses one or more electrons and becomes an electrically charged ion. Losing electrons creates a positive ion, while gaining electrons creates a negative ion.
In more detail
Losing an electron leaves an atom with more protons than electrons, giving it an overall positive charge and turning it into a cation. Gaining an electron gives an atom more electrons than protons, producing a negative charge and forming an anion. Several things can trigger ionization, including heat, light in a process called photoionization, or a collision with another fast-moving particle.
Ionization can also happen when a molecular, or covalent, compound reacts with a polar solvent like water, creating ions that did not previously exist in the pure substance. Hydrogen chloride gas ionizing in water to form H+ and Cl- ions is a clear example of this.
This process differs from dissociation, where an already-ionic solid such as table salt simply separates into ions it already contained once it dissolves. The energy needed to remove the most loosely held electron from a gaseous atom is called its first ionization energy. This value generally rises across a period and falls down a group, following changes in nuclear charge and atomic size.
Instruments called mass spectrometers deliberately ionize sample molecules so their resulting ions can be sorted and identified by mass, making ionization a practical tool as well as a theoretical concept.
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) |
| Periodic trend | Increases across a period, decreases down a group |
A sodium atom loses one electron to form a sodium cation and a free electron, written as Na → Na+ + e-. This process requires 496 kilojoules per mole of sodium, a value known as sodium's first ionization energy. Removing a second electron from the resulting Na+ ion takes far more energy, since that electron comes from a full, more tightly held inner shell.
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, such as NaCl into Na+ and Cl-. 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.
Why is the second ionization energy always higher than the first?
Removing a second electron means pulling it away from an ion that already carries a positive charge, which attracts remaining electrons more strongly. This stronger pull requires more energy than removing the first electron from a neutral atom.