Ionization Energy
Definition and meaning of Ionization Energy in chemistry.
Ionization energy is the minimum energy required to remove the most loosely bound electron from a gaseous atom or ion in its ground state. It is always positive, since energy must be supplied to overcome the attraction between the electron and the nucleus.
In more detail
The first ionization energy (IE1) removes one electron from a neutral atom; the second ionization energy (IE2) removes an electron from the resulting cation, and is always larger than IE1 because the electron is being pulled away from a more positively charged species. Across a period, ionization energy generally increases as nuclear charge rises while shielding stays nearly constant, pulling electrons in more tightly. Down a group, it generally decreases because valence electrons occupy shells farther from the nucleus and are shielded by more inner electrons. Large jumps in successive ionization energies reveal when an electron is removed from a new, lower principal shell, helping confirm an element's number of valence electrons.
Key facts
| Typical Units | kJ/mol or eV/atom |
|---|---|
| General Equation | X(g) → X⁺(g) + e⁻ |
| Periodic Trend | Increases left to right across a period; decreases down a group |
| Field | General Chemistry |
For sodium: Na(g) → Na⁺(g) + e⁻, IE1 = 496 kJ/mol. Removing a second electron from Na⁺ requires far more energy (IE2 = 4562 kJ/mol) because that electron comes from the tightly held, complete inner shell.
Frequently asked questions
Why is ionization energy always a positive value?
Because removing an electron from an atom always requires an input of energy to overcome the nucleus's attractive pull, so the process is endothermic by definition.
Why does the second ionization energy jump so much higher for alkali metals?
After losing one electron, the alkali metal ion has a stable noble-gas configuration; removing a second electron means breaking into a complete, tightly held inner shell, which demands much more energy.