Editing Palladium (section)

==Characteristics==
Palladium belongs to [[group 10 element|group&nbsp;10]] in the periodic table, but the configuration in the outermost electrons is in accordance with [[Hund's rule]]. Electrons that by the [[Madelung rule]] would be expected to occupy the 5[[Atomic orbital|''s'']] instead fill the 4[[Atomic orbital|''d''&nbsp;orbitals]], as it is more energetically favorable to have a completely filled 4d<sup>10</sup> shell instead of the 5s<sup>2</sup> 4d<sup>8</sup> configuration.{{clarify|date=August 2019}}

{| class="wikitable floatleft" border="1" cellpadding="3" cellspacing="0"
|-
![[Atomic number|Z]] !! [[Chemical element|Element]] !! [[Electron shell|No. of electrons/shell]]
|-
| 28 || [[nickel]] || 2, 8, 16, 2 (or 2, 8, 17, 1)
|-
| 46 || palladium || 2, 8, 18, 18, <span title="The valence shell shouldn’t be omitted!">0</span>
|-
| 78 || [[platinum]] || 2, 8, 18, 32, 17, 1
|-
| 110 || [[darmstadtium]] || 2, 8, 18, 32, 32, 16, 2 (predicted)<ref>{{cite book |title=The Chemistry of the Actinide and Transactinide Elements |editor1-last=Morss |editor2-first=Norman M. |editor2-last=Edelstein |editor3-last=Fuger |editor3-first=Jean |last1=Hoffman |first1=Darleane C. |last2=Lee |first2=Diana M. |last3=Pershina |first3=Valeria |chapter=Transactinides and the future elements |publisher=[[Springer Science+Business Media]] |year=2006 |isbn=1-4020-3555-1 |location=Dordrecht, The Netherlands |edition=3rd |page=1722}}</ref>
|}
This 5s<sup>0</sup> configuration, unique in [[period 5 element|period&nbsp;5]], makes palladium the heaviest element having only ''one'' incomplete [[electron shell]], with all shells above it empty.

Palladium has the appearance of a soft silver-white metal that resembles platinum. It is the least dense and has the lowest [[melting point]] of the platinum group metals. It is soft and [[ductile]] when [[Annealing (metallurgy)|annealed]] and is greatly increased in strength and hardness when cold-worked. Palladium dissolves slowly in concentrated [[nitric acid]], in hot, concentrated [[sulfuric acid]], and when finely ground, in [[hydrochloric acid]].<ref name="CRC">{{cite book |author=Hammond, C. R. |chapter=The Elements |title=Handbook of Chemistry and Physics |edition=81st |publisher=CRC press |isbn=978-0-8493-0485-9 |date=2004 |chapter-url-access=registration |chapter-url=https://archive.org/details/crchandbookofche81lide}}</ref> It dissolves readily at room temperature in [[aqua regia]].

Palladium does not react with [[oxygen]] at standard temperature (and thus does not tarnish in [[air]]). Palladium heated to 800&nbsp;°C will produce a layer of palladium(II) oxide (PdO). It may slowly develop a slight brownish coloration over time, likely due to the formation of a surface layer of its monoxide.

Palladium films with defects produced by alpha particle bombardment at low temperature exhibit superconductivity having ''T''<sub>c</sub> = 3.2 K.<ref>B. Strizker, Phys. Rev. Lett., 42, 1769 (1979).</ref>

===Isotopes===
{{Main|Isotopes of palladium}}
Naturally occurring palladium is composed of seven [[isotope]]s, six of which are stable. The most stable [[radioisotope]]s are [[Palladium-107|<sup>107</sup>Pd]] with a [[half-life]] of 6.5 million years (found in nature), [[Pd-103|<sup>103</sup>Pd]] with 17 days, and <sup>100</sup>Pd with 3.63 days. Eighteen other radioisotopes have been characterized with [[atomic weight]]s ranging from 90.94948(64) [[atomic mass unit|u]] (<sup>91</sup>Pd) to 122.93426(64)&nbsp;u (<sup>123</sup>Pd).<ref>{{cite journal |access-date=12 November 2009 |url=http://physics.nist.gov/PhysRefData/Compositions/index.html |title=Atomic Weights and Isotopic Compositions for Palladium (NIST) |journal=NIST |date=2009-08-23}}</ref> These have half-lives of less than thirty minutes, except <sup>101</sup>Pd (half-life: 8.47 hours), <sup>109</sup>Pd (half-life: 13.7 hours), and <sup>112</sup>Pd (half-life: 21 hours).<ref name="NUBASE">{{NUBASE 2003}}</ref>

For isotopes with atomic mass unit values less than that of the most abundant stable isotope, <sup>106</sup>Pd, the primary [[decay mode]] is [[electron capture]] with the primary [[decay product]] being rhodium. The primary mode of decay for those isotopes of Pd with atomic mass greater than 106 is [[beta decay]] with the primary product of this decay being [[silver]].<ref name="NUBASE" />

[[Radiogenic]] <sup>107</sup>Ag is a decay product of <sup>107</sup>Pd and was first discovered in 1978<ref>{{cite journal |title=Evidence for the existence of <sup>107</sup>Pd in the early solar system |journal=Philosophical Transactions of the Royal Society of London, Series A |first3=R. |date=1978 |volume=359 |last3=Hutchison |pages=1079–1082 |doi=10.1098/rsta.2001.0893 |first1=W. R. |last1=Kelly |first2=G. J. |last2=Gounelle |issue=1787 |bibcode=2001RSPTA.359.1991R |s2cid=120355895}}</ref> in the [[Santa Clara, Durango|Santa Clara]]<ref>{{cite web |url=http://mexicogemstones.com/pdf/MexicoMeteorites.pdf |archive-url=https://web.archive.org/web/20060506085632/http://www.mexicogemstones.com/pdf/MexicoMeteorites.pdf |url-status=dead |archive-date=2006-05-06 |title=Mexico's Meteorites |work=mexicogemstones.com}}</ref> meteorite of 1976. The discoverers suggest that the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after a [[nucleosynthetic]] event. <sup>107</sup>Pd versus Ag correlations observed in bodies, which have been melted since accretion of the [[Solar System]], must reflect the presence of short-lived nuclides in the early Solar System.<ref>{{cite journal |title=The isotopic composition of Ag in meteorites and the presence of <sup>107</sup>Pd in protoplanets |journal=Geochimica et Cosmochimica Acta |date=1990 |volume=54 |issue=6 |pages=1729–1743 |doi=10.1016/0016-7037(90)90404-9 |first1=J. H. |last1=Chen |first2=G. J. |last2=Wasserburg |bibcode=1990GeCoA..54.1729C}}</ref> {{chem|107|Pd}} is also produced as a [[fission product]] in spontaneous or induced fission of {{chem|235|U|link=Uranium-235}}. As it is not very mobile in the environment and has a relatively low [[decay energy]], {{chem|107|Pd}} is usually considered to be among the less concerning of the [[long-lived fission products]].