Editing Mass number (section)

==Mass number changes in radioactive decay==
Different types of [[radioactive decay]] are characterized by their changes in mass number as well as [[atomic number]], according to the [[radioactive displacement law of Fajans and Soddy]]. 
For example, [[uranium-238]] usually decays by [[alpha decay]], where the nucleus loses two neutrons and two protons in the form of an [[alpha particle]].  Thus the atomic number and the number of neutrons each decrease by 2 (''Z'': 92 → 90, ''N'': 146 → 144), so that the mass number decreases by 4 (''A'' = 238 → 234); the result is an atom of [[thorium-234]] and an alpha particle ({{nuclide|helium|4|charge=2+}}):<ref name="suchocki">Suchocki, John. ''Conceptual Chemistry'', 2007. Page 119.</ref>

:{| border="0"
|- style="height:2em;"
|{{nuclide|uranium|238}}&nbsp;||→&nbsp;||{{nuclide|thorium|234}}&nbsp;||+&nbsp;||{{nuclide|helium|4|charge=2+}}||
|}

On the other hand, [[carbon-14]] decays by [[beta decay]], whereby one neutron is transmuted into a proton with the emission of an [[electron]] and an [[antineutrino]].  Thus the atomic number increases by 1 (''Z'': 6 → 7) and the mass number remains the same (''A'' = 14), while the number of neutrons decreases by 1 (''N'': 8 → 7).<ref>{{cite book
  | last = Curran
  | first = Greg
  | title = Homework Helpers
  | url = https://archive.org/details/homeworkhelpersc0000curr
  | url-access = registration
  | publisher = Career Press
  | year = 2004
  | pages = [https://archive.org/details/homeworkhelpersc0000curr/page/78 78–79]
  | isbn = 1-56414-721-5 }}</ref>  The resulting atom is [[nitrogen-14]], with seven protons and seven neutrons:

:{| border="0"
|- style="height:2em;"
|{{nuclide|carbon|14}}&nbsp;||→&nbsp;||{{nuclide|nitrogen|14}}&nbsp;||+&nbsp;||{{SubatomicParticle|Electron}}&nbsp;||+&nbsp;||{{SubatomicParticle|Electron Antineutrino}}
|}

Beta decay is possible because different [[isobar (nuclide)|isobars]]<ref name="isobar">Atoms with the same mass number.</ref> have mass differences on the order of a few [[electron mass]]es. If possible, a nuclide will undergo beta decay to an adjacent isobar with lower mass. In the absence of other decay modes, a cascade of beta decays terminates at the [[beta-stability line|isobar with the lowest atomic mass]].

Another type of radioactive decay without change in mass number is emission of a [[gamma ray]] from a [[nuclear isomer]] or [[metastable]] excited state of an atomic nucleus. Since all the protons and neutrons remain in the nucleus unchanged in this process, the mass number is also unchanged.