# Is electronvolt a mass or an energy unit?

I always thought electronvolt was an unit of energy and I knew its definition, but in these days I got some doubts because I saw 2 times it was used as an unit of mass: in my school textbook, in an article about the 4 fundamental forces, it was used as a mass unit; and in a famous magazine I read the same thing (same arguments)

In physics, the electron volt (symbol eV; also written electronvolt[1][2]) is a unit of energy equal to...

particularly there was a table about sub-atomic particles with .... spin | charge | mass(GeV) | ...

• – Řídící Oct 18 '13 at 22:36

Strictly speaking it's a unit of energy. But using $m=\frac{E}{c^2}$, you can convert energy into mass. Operating, we get $1{\rm\,eV}/c^2 =1.78\cdot 10^{-36}\rm{\,kg}$. (The $c^2$ is usually ommited.)

It is absolutely a unit of energy, though your confusion may arise due to the usage of what's known as geometrized units, where c the speed of light (in addition to the gravitational constant G) is set to unity. This is typically done in General Relativity. In this system, mass and energy have exactly the same units - they are two sides of the same coin.

It is an energy unit, however you can use it as a mass unit too because of the famous Einstein relation E=m*c^2. If you are given a mass in eV, you just have to use that formula to get the corresponding mass in real mass units.