<P> Nuclei are bound together by the residual strong force (nuclear force). The residual strong force is a minor residuum of the strong interaction which binds quarks together to form protons and neutrons . This force is much weaker between neutrons and protons because it is mostly neutralized within them, in the same way that electromagnetic forces between neutral atoms (such as van der Waals forces that act between two inert gas atoms) are much weaker than the electromagnetic forces that hold the parts of the atoms together internally (for example, the forces that hold the electrons in an inert gas atom bound to its nucleus). </P> <P> The nuclear force is highly attractive at the distance of typical nucleon separation, and this overwhelms the repulsion between protons due to the electromagnetic force, thus allowing nuclei to exist . However, the residual strong force has a limited range because it decays quickly with distance (see Yukawa potential); thus only nuclei smaller than a certain size can be completely stable . The largest known completely stable nucleus (i.e. stable to alpha, beta, and gamma decay) is lead - 208 which contains a total of 208 nucleons (126 neutrons and 82 protons). Nuclei larger than this maximum are unstable and tend to be increasingly short - lived with larger numbers of nucleons . However, bismuth - 209 is also stable to beta decay and has the longest half - life to alpha decay of any known isotope, estimated at a billion times longer than the age of the universe . </P> <P> The residual strong force is effective over a very short range (usually only a few femtometres (fm); roughly one or two nucleon diameters) and causes an attraction between any pair of nucleons . For example, between protons and neutrons to form (NP) deuteron, and also between protons and protons, and neutrons and neutrons . </P> <P> The effective absolute limit of the range of the strong force is represented by halo nuclei such as lithium - 11 or boron - 14, in which dineutrons, or other collections of neutrons, orbit at distances of about 6986100000000000000 ♠ 10 fm (roughly similar to the 6985800000000000000 ♠ 8 fm radius of the nucleus of uranium - 238). These nuclei are not maximally dense . Halo nuclei form at the extreme edges of the chart of the nuclides--the neutron drip line and proton drip line--and are all unstable with short half - lives, measured in milliseconds; for example, lithium - 11 has a half - life of 6997880000000000000 ♠ 8.8 ms . </P>

A particle that orbits the nucleus in an atom is called a(n