Electron Volt
Definition and meaning of Electron Volt in chemistry.
Electron volt (eV) is a unit of energy equal to the kinetic energy gained by a single electron when it is accelerated through an electric potential difference of one volt in a vacuum. It is used instead of the joule when dealing with the very small energies typical of individual atoms, molecules, or subatomic particles.
In more detail
Because 1 eV equals the elementary charge (in coulombs) multiplied by one volt, it has an exact value in SI units: 1 eV = 1.602176634 × 10⁻19 J. Chemists favor the electron volt for reporting quantities like ionization energies, electron affinities, and orbital energy levels, since these values come out as small, easy-to-read numbers rather than tiny fractions of a joule. To compare with molar thermodynamic quantities in kJ/mol, an eV-per-particle value is multiplied by Avogadro's constant, giving the conversion 1 eV = 96.485 kJ/mol.
Key facts
| Field | Physical Chemistry |
|---|---|
| Symbol | eV |
| SI equivalent | 1 eV = 1.602176634 × 10⁻19 J (exact) |
| Molar equivalent | 1 eV ≈ 96.485 kJ/mol |
The first ionization energy of a hydrogen atom is 13.6 eV, which converts to about 1312 kJ/mol, the energy needed to remove one mole of electrons from one mole of gaseous hydrogen atoms.
Frequently asked questions
Why do chemists use electron volts instead of joules?
Atomic and molecular energies (ionization energies, orbital energies, photon energies in spectroscopy) are extremely small in joules, so the electron volt gives more convenient, human-scale numbers, such as 13.6 eV for hydrogen's ionization energy.
How is an electron volt converted to kJ/mol?
Multiply the eV value by Avogadro's constant and the elementary charge to get joules per mole, then divide by 1000; the shortcut conversion factor is 1 eV = 96.485 kJ/mol.