First Ionization Potential
Definition and meaning of First Ionization Potential in chemistry.
First ionization potential (first ionization energy) is the minimum energy needed to remove the most loosely bound electron from one mole of neutral, gaseous atoms in their ground state, producing gaseous +1 cations.
In more detail
It is always endothermic, since energy must overcome the attraction between the nucleus and the departing electron. Values generally increase left to right across a period, as increasing effective nuclear charge pulls valence electrons in more tightly, and decrease down a group, as electrons occupy shells farther from the nucleus with more shielding. Irregularities occur where removing an electron relieves extra stability, such as at half-filled or filled subshells: oxygen's first ionization energy is lower than nitrogen's because removing one electron eliminates electron-electron repulsion in a doubly occupied 2p orbital.
Key facts
| Typical units | kJ/mol or eV (1 eV/atom ≈ 96.5 kJ/mol) |
|---|---|
| Periodic trend | Increases across a period; decreases down a group |
| Example value | Na: 496 kJ/mol; He: 2372 kJ/mol (highest of all elements) |
| Field | General Chemistry |
For sodium, Na(g) → Na⁺(g) + e⁻ requires 496 kJ/mol, far less than the 4560 kJ/mol needed to remove a second electron from the stable noble-gas configuration of Na⁺.
Frequently asked questions
Why does helium have the highest first ionization energy?
Helium's small atomic radius and high effective nuclear charge on its two 1s electrons, with no inner shielding, make its electrons unusually tightly bound.
Is ionization potential the same as ionization energy?
Yes; ionization potential is the traditional name (from early electron-impact experiments measuring a voltage) for the same quantity now usually called ionization energy.