Matter And Antimatter:

The proton, neutron, and electron each has its own nemesis particle which occurs in the form of antimatter. Such particles are known as antiparticles. Antiparticle for the proton is the antiproton; for neutron it is the antineutron; for electron it is the positron. The antiproton consist of the same mass as the proton, though in a negative kind of way, and it has a negative electric charge which is equivalent but opposite to the positive electric charge of the proton. The antineutron has similar mass as the neutron, though again in a negative logic. Neither the neutron nor the antineutron has any electric charge. Positron has the same mass as the electron, though in a negative sense, and it is positively charged to a level equivalent to the negative charge on an electron.

You may have read or seen in science-fiction novels and movies that whenever a particle of matter collides with its nemesis, they eradicate each other. This is true. What, accurately, does this mean? Actually, the particles do not just disappear from the cosmos, though they change from matter into energy. The collective mass of the particle and the antiparticle is liberated entirely according to the same Einstein formula which applies in nuclear reactions:

E = (m₊ + m_- ) c²

Here E is the energy in joules; m₊ is the mass of the particle in kilograms, m_- is the mass of the antiparticle in kilograms, and c is the speed of light squared, that, as you remember, is around equal to 9 x 10¹⁶ m²/s².