Editing Chromium (section)

=== Bulk ===
[[File:Chromium.jpg|thumb|left|Sample of chromium metal]]
Chromium is the third hardest element after [[carbon]] ([[diamond]]) and [[boron]]. Its [[Mohs scale of mineral hardness|Mohs hardness]] is 8.5, which means that it can scratch samples of [[quartz]] and [[topaz]], but can be scratched by [[corundum]]. Chromium is highly resistant to [[tarnish]]ing, which makes it useful as a metal that preserves its outermost layer from [[corrosion|corroding]], unlike other metals such as [[copper]], [[magnesium]], and [[aluminium]].

Chromium has a [[melting point]] of 1907&nbsp;°C (3465&nbsp;°F), which is relatively low compared to the majority of transition metals. However, it still has the second highest melting point out of all the [[period 4 element]]s, being topped by [[vanadium]] by 3&nbsp;°C (5&nbsp;°F) at 1910&nbsp;°C (3470&nbsp;°F). The [[boiling point]] of 2671&nbsp;°C (4840&nbsp;°F), however, is comparatively lower, having the fourth lowest boiling point out of the [[Period 4]] [[transition metal]]s alone behind [[copper]], [[manganese]] and [[zinc]].<ref group=note>The melting/boiling point of transition metals are usually higher compared to the alkali metals, alkaline earth metals, and nonmetals, which is why the range of elements compared to chromium differed between comparisons</ref> The [[electrical resistivity and conductivity|electrical resistivity]] of chromium at 20&nbsp;°C is 125 [[ohm|nanoohm]]-[[meter]]s.

Chromium has a high [[specular reflection]] in comparison to other transition metals. In [[infrared]], at 425&nbsp;[[micrometre|μm]], chromium has a maximum reflectance of about 72%, reducing to a minimum of 62% at 750&nbsp;μm before rising again to 90% at 4000&nbsp;μm.<ref name = "NIST specular reflection" /> When chromium is used in [[stainless steel]] alloys and [[polishing|polished]], the specular reflection decreases with the inclusion of additional metals, yet is still high in comparison with other alloys. Between 40% and 60% of the visible spectrum is reflected from polished stainless steel.<ref name = "NIST specular reflection" /> The explanation on why chromium displays such a high turnout of reflected [[photon]] waves in general, especially the 90% in infrared, can be attributed to chromium's magnetic properties.<ref name="ISU infrared">{{cite journal|last1 = Lind|first1 = Michael Acton|title = The infrared reflectivity of chromium and chromium-aluminium alloys|url = https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=6216&context=rtd|website = Iowa State University Digital Repository|publisher = Iowa State University|date = 1972|access-date = 4 November 2018|bibcode = 1972PhDT........54L|archive-date = 30 September 2021|archive-url = https://web.archive.org/web/20210930205235/https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=6216&context=rtd|url-status = live}}</ref> Chromium has unique magnetic properties; it is the only elemental solid that shows [[antiferromagnetic]] ordering at room temperature and below. Above 38&nbsp;°C, its magnetic ordering becomes [[paramagnetic]].<ref name="fawcett" /> The antiferromagnetic properties, which cause the chromium atoms to temporarily [[ionization|ionize]] and bond with themselves, are present because the body-centric cubic's magnetic properties are disproportionate to the [[Crystal structure|lattice periodicity]]. This is due to the magnetic moments at the cube's corners and the unequal, but antiparallel, cube centers.<ref name="ISU infrared" /> From here, the frequency-dependent [[relative permittivity]] of chromium, deriving from [[Maxwell's equations]] and chromium's [[antiferromagnetism]], leaves chromium with a high infrared and visible light reflectance.<ref name="ISU optical">{{cite journal|last1 = Bos|first1 = Laurence William|title = Optical properties of chromium-manganese alloys|url = https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=4816&context=rtd|website = Iowa State University Digital Repository|publisher = Iowa State University|date = 1969|access-date = 4 November 2018|bibcode = 1969PhDT.......118B|archive-date = 30 September 2021|archive-url = https://web.archive.org/web/20210930205225/https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=4816&context=rtd|url-status = live}}</ref>

==== Passivation ====
Chromium metal in air is [[passivation (chemistry)|passivated]]: it forms a thin, protective surface layer of chromium oxide with the [[corundum_(structure)|corundum]] structure. Passivation can be enhanced by short contact with [[oxidizing acid]]s like [[nitric acid]]. Passivated chromium is stable against acids. Passivation can be removed with a strong [[reducing agent]] that destroys the protective oxide layer on the metal. Chromium metal treated in this way readily dissolves in weak acids.<ref name = "HollemanAF" />

The surface [[chromium(III) oxide|chromia]] {{chem2|Cr2O3}} scale, is adherent to the metal. In contrast, iron forms a more porous oxide which is weak and flakes easily and exposes fresh metal to the air, causing continued [[rust]]ing. At room temperature, the chromia scale is a few atomic layers thick, growing in thickness by outward [[Atomic_diffusion|diffusion]] of metal ions across the scale. Above 950 °C volatile [[chromium trioxide]] {{chem2|CrO3}} forms from the chromia scale, limiting the scale thickness and oxidation protection.<ref>{{Cite journal|title = The oxidation of alloys|last = Wallwork|first = GR|date = 1976|journal = Reports on Progress in Physics|volume = 39|pages = 401–485|doi = 10.1088/0034-4885/39/5/001|issue = 5|bibcode = 1976RPPh...39..401W | s2cid=250853920 }}</ref>

Chromium, unlike iron and nickel, does not suffer from [[hydrogen embrittlement]]. However, it does suffer from nitrogen [[embrittlement]], reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal parts.<ref>{{Cite book|url = https://books.google.com/books?id=CGMrAAAAYAAJ|title = High-temperature oxidation-resistant coatings: coatings for protection from oxidation of superalloys, refractory metals, and graphite|author = National Research Council (U.S.). Committee on Coatings|publisher = National Academy of Sciences|date = 1970|isbn = 978-0-309-01769-5|access-date = 5 June 2020|archive-date = 10 June 2024|archive-url = https://web.archive.org/web/20240610050309/https://books.google.com/books?id=CGMrAAAAYAAJ|url-status = live}}</ref>