Nuclear drip line
In nuclear physics, the boundaries for nuclear particle-stability are called drip lines. Atomic nuclei contain both protons and neutrons—the number of protons defines the identity of that element (ie, carbon always has 6 protons), but the number of neutrons within that element may vary (carbon-12 and its isotope carbon-13, for example). The number of isotopes each element may have is visually represented by plotting boxes, each of which represents a unique nuclear species, on a graph with the number of neutrons increasing on the abscissa (X axis) and number of protons increasing along the ordinate (Y axis). The resulting chart is commonly referred to as the table of nuclides, and is to nuclear physics what the periodic table of the elements is to chemistry.An arbitrary combination of protons and neutrons does not necessarily yield a stable nucleus. One can think of moving up and/or to the right across the nuclear chart by adding one type of nucleon (i.e. a proton or neutron, both called nucleons) to a given nucleus. However, adding nucleons one at a time to a given nucleus will eventually lead to a newly formed nucleus that immediately decays by emitting a proton (or neutron). Colloquially speaking, the nucleon has 'leaked' or 'dripped' out of the nucleus, hence giving rise to the term ""drip line"". Drip lines are defined for protons, neutrons, and alpha particles, and these all play important roles in nuclear physics. The nucleon drip lines are at the extreme of the proton-to-neutron ratio: at p:n ratios at or beyond the driplines, no stable nuclei can exist. The location of the neutron drip line is not well known for most of the nuclear chart, whereas the proton and alpha driplines have been measured for a wide range of elements. The nucleons drip out of such unstable nuclei for the same reason that water drips from a leaking faucet: in the water case, there is a lower potential available that is great enough to overcome surface tension and so produces a droplet; in the case of nuclei, the emission of a particle from a nucleus, against the strong nuclear force, leaves the total potential of the nucleus and the emitted particle in a lower state. Because nucleons are quantized, only integer values are plotted on the table of isotopes; this indicates that the drip line is not linear but instead looks like a step function up close.