Editing Copper (section)

===Physical===
[[File:Cu-Scheibe.JPG|thumb|left|upright=0.7|A copper disc (99.95% pure) made by [[continuous casting]]; [[industrial etching|etched]] to reveal [[crystallite]]s]]

[[File:Molten copper in bright sunlight.gif|upright=0.7|thumb|left|Copper just above its melting point keeps its pink luster color when enough light outshines the orange [[incandescence]] color.]]

Copper, [[silver]], and [[gold]] are in [[group 11 element|group 11]] of the periodic table; these three metals have one s-orbital electron on top of a filled d-[[electron shell]] and are characterized by high [[ductility]], and electrical and thermal conductivity. The filled d-shells in these elements contribute little to interatomic interactions, which are dominated by the s-electrons through [[metallic bond]]s. Unlike metals with incomplete d-shells, metallic bonds in copper are lacking a [[covalent bond|covalent]] character and are relatively weak. This observation explains the low [[hardness]] and high ductility of [[monocrystalline|single crystals]] of copper.<ref name="b1">{{cite book|first1=George L. |last1=Trigg|first2=Edmund H. |last2=Immergut|title=Encyclopedia of Applied Physics|url=https://books.google.com/books?id=sVQ5RAAACAAJ|access-date=2 May 2011|year=1992|publisher=VCH |isbn=978-3-527-28126-8|pages=267–272|volume=4: Combustion to Diamagnetism}}</ref> At the macroscopic scale, introduction of extended defects to the [[crystal lattice]], such as grain boundaries, hinders flow of the material under applied stress, thereby increasing its hardness. For this reason, copper is usually supplied in a fine-grained [[polycrystalline]] form, which has greater strength than monocrystalline forms.<ref>{{cite book|last1 = Smith |first1=William F.|last2 = Hashemi |first2=Javad|name-list-style = amp |title = Foundations of Materials Science and Engineering|page = 223|publisher = McGraw-Hill Professional|date= 2003|isbn = 978-0-07-292194-6}}</ref>

The softness of copper partly explains its high electrical conductivity ({{val|59.6|e=6|u=[[Siemens (unit)|S]]/m}}) and high thermal conductivity, second highest (second only to silver) among pure metals at room temperature.<ref name="CRC">{{cite book|last = Hammond |first=C. R.|title = The Elements, in Handbook of Chemistry and Physics|edition = 81st|publisher = CRC Press|isbn = 978-0-8493-0485-9|date = 2004|url = https://archive.org/details/crchandbookofche81lide}}</ref> This is because the resistivity to electron transport in metals at room temperature originates primarily from scattering of electrons on thermal vibrations of the lattice, which are relatively weak in a soft metal.<ref name="b1" /> The maximum possible current density of copper in open air is approximately {{val|3.1|e=6|u=A/m<sup>2</sup>}}, above which it begins to heat excessively.<ref>{{cite book|author=Resistance Welding Manufacturing Alliance |title=Resistance Welding Manual|date=2003|publisher=Resistance Welding Manufacturing Alliance|isbn=978-0-9624382-0-2|edition=4th|pages=18–12}}</ref>

Copper is one of a few metallic elements with a natural color other than gray or silver.<ref>{{Cite book|last1 = Chambers|first1 = William|last2 = Chambers|first2 = Robert|title = Chambers's Information for the People|publisher = W. & R. Chambers|date = 1884|volume = L|page = 312|edition = 5th|url = https://books.google.com/books?id=eGIMAAAAYAAJ|isbn = 978-0-665-46912-1}}</ref> Pure copper is orange-red and acquires a reddish [[tarnish]] when exposed to air. This is due to the low [[plasma frequency]] of the metal, which lies in the red part of the visible spectrum, causing it to absorb the higher-frequency green and blue colors.<ref>{{cite web |last1=Ramachandran |first1=Harishankar |title=Why is Copper Red? |url=http://ee.iitm.ac.in/~hsr/ec301/copper.pdf |website=[[IIT Madras]] |access-date=27 December 2022 |date=14 March 2007}}</ref>

As with other metals, if copper is put in contact with another metal in the presence of an [[electrolyte]], [[galvanic corrosion]] will occur.<ref>{{cite web|title=Galvanic Corrosion|url=http://www.corrosion-doctors.org/Forms-galvanic/galvanic-corrosion.htm|work=Corrosion Doctors|access-date=29 April 2011}}</ref>