Editing Nuclear reaction (section)

==Energy conservation==
[[Kinetic energy]] may be released during the course of a reaction ([[chemical reaction#Exothermic reactions|exothermic reaction]]) or kinetic energy may have to be supplied for the reaction to take place ([[chemical reaction#Endothermic reactions|endothermic reaction]]). This can be calculated by reference to a table of very accurate particle rest masses,<ref>{{cite web|url=https://www.nist.gov/pml/atomic-weights-and-isotopic-compositions-relative-atomic-masses|title=Atomic Weights and Isotopic Compositions with Relative Atomic Masses|first=Curt|last=Suplee|date=23 August 2009|website=NIST}}</ref> as follows: according to the reference tables, the {{nuclide|Lithium|6}} nucleus has a [[standard atomic weight]] of 6.015 [[atomic mass unit]]s (abbreviated [[u]]), the deuterium has 2.014 u, and the helium-4 nucleus has 4.0026 u. Thus:
* the sum of the rest mass of the individual nuclei = 6.015 + 2.014 = 8.029 u;
* the total rest mass on the two helium-nuclei = 2 × 4.0026 = 8.0052 u;
* missing rest mass = 8.029 – 8.0052 = 0.0238 atomic mass units.

In a nuclear reaction, the total [[conservation of energy|(relativistic) energy is conserved]]. The "missing" rest mass must therefore reappear as kinetic energy released in the reaction; its source is the nuclear [[binding energy]]. Using Einstein's [[mass-energy equivalence]] formula ''E''&nbsp;=&nbsp;''mc''<sup>2</sup>, the amount of energy released can be determined. We first need the energy equivalent of one [[atomic mass unit]]:

{{block indent|1=1&nbsp;u&nbsp;''c''<sup>2</sup>&nbsp;=&nbsp;(1.66054&nbsp;×&nbsp;10<sup>−27</sup>&nbsp;kg)&nbsp;×&nbsp;(2.99792&nbsp;×&nbsp;10<sup>8</sup>&nbsp;m/s)<sup>2</sup>
{{block indent|1= =&nbsp;1.49242&nbsp;×&nbsp;10<sup>−10</sup>&nbsp;kg&nbsp;(m/s)<sup>2</sup>}}
{{block indent|1==&nbsp;1.49242&nbsp;×&nbsp;10<sup>−10</sup>&nbsp;[[Joule|J]]}}
{{block indent|1= =&nbsp;931.49&nbsp;MeV (1&nbsp;MeV&nbsp;=&nbsp;1.602176634×10<sup>−13</sup>&nbsp;J),}}}}
{{block indent|1= so 1&nbsp;u&nbsp;''c''<sup>2</sup>&nbsp;=&nbsp;931.49&nbsp;MeV.}}

Hence, the energy released is 0.0238 × 931 MeV = 22.2 [[MeV]].

Expressed differently: the mass is reduced by 0.3%, corresponding to 0.3% of 90&nbsp;PJ/kg is 270&nbsp;TJ/kg.

This is a large amount of energy for a nuclear reaction; the amount is so high because the binding energy per [[nucleon]] of the helium-4 nucleus is unusually high because the He-4 nucleus is "[[magic number (physics)|doubly magic]]". (The He-4 nucleus is unusually stable and tightly bound for the same reason that the helium atom is inert: each pair of protons and neutrons in He-4 occupies a filled '''1s''' [[nuclear orbital]] in the same way that the pair of electrons in the helium atom occupy a filled '''1s''' [[atomic orbital|electron orbital]]). Consequently, alpha particles appear frequently on the right-hand side of nuclear reactions.

The energy released in a nuclear reaction can appear mainly in one of three ways:
*kinetic energy of the product particles (fraction of the kinetic energy of the charged nuclear reaction products can be directly converted into electrostatic energy);<ref>{{cite journal|last1=Shinn|first1=E.|last2=Et.|first2=al.|title=Nuclear energy conversion with stacks of graphene nanocapacitors|journal=Complexity|date=2013|volume=18|issue=3|pages=24–27|doi=10.1002/cplx.21427|bibcode=2013Cmplx..18c..24S}}</ref>
*emission of very high energy [[photon]]s, called [[gamma ray]]s;
*some energy may remain in the nucleus, as a [[metastable]] [[energy level]].

When the product nucleus is metastable, this is indicated by placing an [[asterisk]] ("*") next to its atomic number. This energy is eventually released through [[nuclear decay]].

A small amount of energy may also emerge in the form of [[X-ray]]s. Generally, the product nucleus has a different atomic number, and thus the configuration of its [[electron shell]]s is wrong. As the electrons rearrange themselves and drop to lower energy levels, internal transition X-rays (X-rays with precisely defined [[emission line]]s) may be emitted.