Among the force carrying particles it is only the three vector bosons of the weak interaction that have mass. The others, the photon and the eight gluons, are all massless. The quarks and the leptons all have different masses. The three neutral leptons, the neutrinos, have a very small mass. Until recently it was believed that they were massless.
The masses of the particles are given in the unit electron volt, which is written as eV.
Energy takes the form as, among other things, kinetic energy and mass. In 1905 Albert Einstein derived an equation for the relation between mass and energy: E = mc2, where E = energy, m = mass and c = the speed of light in vacuum. The equation says how much energy that is needed to create a particle of mass m. Conversely mass can be converted to energy, for example when a particle and an anti-particle are destroyed when they meet ("annihilation"), or when an unstable particle decays into other particles and a small part of the mass becomes energy.
It is difficult to give unambiguous values of the masses of the quarks. The reason for this is that they cannot be separated so that a measurement of the mass in the traditional sense can be made. In the table to the left the "naked masses" of the quarks have been given.