Examples
- A proton and an electron can move separately; the total center-of-mass energy is positive, and such a pair of particles can be described as an ionized atom. Once the electron starts to "orbit" the proton, the energy becomes negative, and a bound state – namely the hydrogen atom – is formed. Only the lowest energy bound state, the ground state is stable. The other excited states are unstable and will decay into bound states with less energy by emitting a photon.
- A nucleus is a bound state of protons and neutrons (nucleons).
- A positronium "atom" is an unstable bound state of an electron and a positron. It decays into photons.
- The proton itself is a bound state of three quarks (two up and one down; one red, one green and one blue). However, unlike the case of the hydrogen atom, the individual quarks can never be isolated. See confinement.
Read more about this topic: Bound State
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