Editing Atom (section)

=== Nucleus ===
{{Main|Atomic nucleus}}
[[File:Binding energy curve - common isotopes.svg|thumb|The [[binding energy]] needed for a nucleon to escape the nucleus, for various isotopes]]<!-- A brief explanation is provided here because 'binding energy' is not explained until the end of the section. -->

All the bound protons and neutrons in an atom make up a tiny [[atomic nucleus]], and are collectively called [[nucleon]]s. The radius of a nucleus is approximately equal to <math>1.07 \sqrt[3]{A}</math>&nbsp;[[femtometre]]s, where <math>A</math> is the total number of nucleons.<ref>{{cite book|last=Jevremovic|first=Tatjana|year=2005|title=Nuclear Principles in Engineering|url=https://archive.org/details/nuclearprinciple00jevr_450|url-access=limited|publisher=Springer|isbn=978-0-387-23284-3|oclc=228384008|page=[https://archive.org/details/nuclearprinciple00jevr_450/page/n83 63]}}</ref> This is much smaller than the radius of the atom, which is on the order of 10<sup>5</sup>&nbsp;fm. The nucleons are bound together by a short-ranged attractive potential called the [[residual strong force]]. At distances smaller than 2.5&nbsp;fm this force is much more powerful than the [[electrostatic force]] that causes positively charged protons to repel each other.<ref>{{cite book|last1=Pfeffer|first1=Jeremy I.|last2=Nir|first2=Shlomo|year=2000|title=Modern Physics: An Introductory Text|publisher=Imperial College Press|isbn=978-1-86094-250-1|oclc=45900880|pages=330–336}}</ref>

Atoms of the same [[chemical element|element]] have the same number of protons, called the [[atomic number]]. Within a single element, the number of neutrons may vary, determining the [[isotope]] of that element. The total number of protons and neutrons determine the [[nuclide]]. The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing [[radioactive decay]].<ref name=wenner2007 />

The proton, the electron, and the neutron are classified as [[fermion]]s. Fermions obey the [[Pauli exclusion principle]] which prohibits ''[[identical particles|identical]]'' fermions, such as multiple protons, from occupying the same quantum state at the same time. Thus, every proton in the nucleus must occupy a quantum state different from all other protons, and the same applies to all neutrons of the nucleus and to all electrons of the electron cloud.<ref name="raymond" />

A nucleus that has a different number of protons than neutrons can potentially drop to a lower energy state through a radioactive decay that causes the number of protons and neutrons to more closely match. As a result, atoms with matching numbers of protons and neutrons are more stable against decay, but with increasing atomic number, the mutual repulsion of the protons requires an increasing proportion of neutrons to maintain the stability of the nucleus.<ref name="raymond" />

[[File:Wpdms physics proton proton chain 1.svg|right|thumb|upright|Illustration of a nuclear fusion process that forms a deuterium nucleus, consisting of a proton and a neutron, from two protons. A [[positron]] (e<sup>+</sup>)—an [[antimatter]] electron—is emitted along with an electron [[neutrino]].]]

The number of protons and neutrons in the atomic nucleus can be modified, although this can require very high energies because of the strong force. [[Nuclear fusion]] occurs when multiple atomic particles join to form a heavier nucleus, such as through the energetic collision of two nuclei. For example, at the core of the Sun protons require energies of 3 to 10&nbsp;keV to overcome their mutual repulsion—the [[coulomb barrier]]—and fuse together into a single nucleus.<ref name=mihos2002 /> [[Nuclear fission]] is the opposite process, causing a nucleus to split into two smaller nuclei—usually through radioactive decay. The nucleus can also be modified through bombardment by high energy subatomic particles or photons. If this modifies the number of protons in a nucleus, the atom changes to a different chemical element.<ref name=lbnl20070330 /><ref name=makhijani_saleska2001 />

If the mass of the nucleus following a fusion reaction is less than the sum of the masses of the separate particles, then the difference between these two values can be emitted as a type of usable energy (such as a [[gamma ray]], or the kinetic energy of a [[beta particle]]), as described by [[Albert Einstein]]'s [[mass–energy equivalence]] formula, ''E = mc<sup>2</sup>'', where ''m'' is the mass loss and ''c'' is the [[speed of light]]. This deficit is part of the [[binding energy]] of the new nucleus, and it is the non-recoverable loss of the energy that causes the fused particles to remain together in a state that requires this energy to separate.<ref>{{cite book|last1=Shultis|first1=J. Kenneth|last2=Faw|first2=Richard E.|title=Fundamentals of Nuclear Science and Engineering|year=2002|publisher=CRC Press|isbn=978-0-8247-0834-4|oclc=123346507|pages=10–17}}</ref>

The fusion of two nuclei that create larger nuclei with lower atomic numbers than [[iron]] and [[nickel]]—a total nucleon number of about 60—is usually an [[exothermic reaction|exothermic process]] that releases more energy than is required to bring them together.<ref name=ajp63_7_653 /> It is this energy-releasing process that makes nuclear fusion in [[star]]s a self-sustaining reaction.  For heavier nuclei, the binding energy per [[nucleon]] begins to decrease. That means that a fusion process producing a nucleus that has an atomic number higher than about 26, and a [[mass number]] higher than about 60, is an [[endothermic reaction|endothermic process]]. Thus, more massive nuclei cannot undergo an energy-producing fusion reaction that can sustain the [[hydrostatic equilibrium]] of a star.<ref name="raymond" />