1.1.3: Atomic Particles

The particles which make up the atom are made up of quarks and leptons.


Protons are baryons, a type of hadron, made up of two up quarks and a single down quark $2(\frac23) + \frac{-1}{3} = 1$. This gives the proton a charge of +1.
Protons exist in the nucleus either alone (hydrogen atom) or with neutrons. Protons repel each other due to their charge, but this may be overcome by the residual strong force which converts some of their mass to energy, binding them together with neutrons and each other.


Neutrons are also baryons, but made up of a single up quark and two down quarks $\frac23 + 2(\frac{-1}{3}) = 0$. This gives neutrons a neutral charge.
Neutrons exist in the nucleus with protons. Due to their lack of charge, they are not repelled from each other or from protons. They may help to keep the nucleus intact by contributing to the binding energy without increasing the repulsive electromagnetic force.


Electrons are the only type of charged lepton in normal existence. Electrons are negatively charged and are attracted to the nucleus of the atom due to its positive charge.


Positrons are the antimatter equivalent of electrons. They carry a positive charge. When a positron and electron meet they may annihalate, converting the mass of each into energy (0.511 MeV is the energy (through E = mc^2) of an electron or positron at rest).