Joules (J) and electron volts (eV) are two common units of energy. This example problem demonstrates how to convert joules to electron volts.

When working with energy values typical for the atomic scale, the joule is too large of a unit to be effective. The electron volt is a unit of energy suited to energies involved in atomic studies. The electron volt is defined as the total amount of kinetic energy gained by an unbound electron as it is accelerated through a potential difference of one volt.

The conversion factor is 1 electron volt (eV) = 1.602 x 10^{-19} J**Problem:**

The ionization energy of a hydrogen atom is 2.195 x 10^{-18} J. What is this energy in electron volts?**Solution:**

x eV = 2.195 x 10^{-18} J x 1 ev/1.602 x 10^{-19} J x eV = 13.7 eV

**Answer:**

The ionization energy of a hydrogen atom is 13.7 eV.