Editing Exotic atom (section)

=== Muonic helium (Hydrogen-4.1) ===
The symbol <sup>4.1</sup>H (Hydrogen-4.1) has been used to describe the exotic atom muonic helium (<sup>4</sup>He-μ), which is like [[helium-4]] in having two [[proton]]s and two [[neutron]]s.<ref name=“Fleming”>
{{Cite journal
  | title = Kinetic Isotope Effects for the Reactions of Muonic Helium and Muonium with H<sub>2</sub>
  | journal = Science | volume = 331 | issue = 6016 | pages = 448–450 | date = 28 Jan 2011
  | doi = 10.1126/science.1199421 
  | last1 = Fleming | first1 = D. G.
  | last2 = Arseneau | first2 = D. J.
  | last3 = Sukhorukov | first3 = O.
  | last4 = Brewer | first4 = J. H.
  | last5 = Mielke | first5 = S. L.
  | last6 = Schatz | first6 = G. C.
  | last7 = Garrett | first7 = B. C.
  | last8 = Peterson | first8 = K. A.
  | last9 = Truhlar | first9 = D. G.
  | pmid = 21273484 | bibcode = 2011Sci...331..448F | s2cid = 206530683
 }}</ref> However one of its [[electron]]s is replaced by a [[muon]], which also has charge –1.  Because the muon's orbital radius is less than {{sfrac|1|200th}} the electron's orbital radius (due to the mass ratio), the muon can be considered as a part of the nucleus. The atom then has a [[atomic nucleus|nucleus]] with two protons, two neutrons and one muon, with total nuclear charge +1 (from two protons and one muon) and only one electron outside, so that it is effectively an isotope of hydrogen instead of an isotope of helium. A muon's weight is approximately 0.1 [[atomic mass unit|Da]] so the isotopic mass is 4.1. Since there is only one electron outside the nucleus, the hydrogen-4.1 atom can react with other atoms. Its chemical behavior behaves more like a hydrogen atom than an inert helium atom. <ref name=“Fleming”/><ref>
{{Cite journal
 | title = Muonic alchemy: Transmuting elements with the inclusion of negative muons
 | journal = Chemical Physics Letters
 | volume = 539
 | pages = 209–221
 | doi = 10.1016/j.cplett.2012.04.062
 | last1 = Moncada | first1 = F.
 | last2 = Cruz | first2 = D.
 | last3 = Reyes | first3 = A
 |bibcode = 2012CPL...539..209M | year = 2012
}}</ref><ref>
{{Cite journal
 | title = Electronic properties of atoms and molecules containing one and two negative muons
 | journal = Chemical Physics Letters
 | volume = 570 | pages = 16–21
 | date =10 May 2013
 | doi = 10.1016/j.cplett.2013.03.004
 | last1 = Moncada | first1 = F.
 | last2 = Cruz | first2 = D.
 | last3 = Reyes | first3 = A.
 |bibcode = 2013CPL...570...16M
}}</ref>