The chemical element chromium has the atomic number 24 and the symbol Cr. It is the first element of group 6. It is a steel gray, shiny, hard and brittle transition metal. The high corrosion resistance and hardness of chromium metal make it valuable. The discovery that by adding metallic chromium to make stainless steel, the steel could be made extremely resistant to corrosion and discoloration was an important advance in steelmaking. 85% of commercial use consists of chrome plated (chrome electroplated) stainless steel. Chromium is highly prized for its ability to be finely polished while remaining tarnish resistant. About 70% of the visible spectrum and about 90% of infrared light is reflected by the polished chrome.
Due to the vibrant colors of many chromium compounds, the name of the element is derived from the Greek word chrma, meaning color.
In the industrial production of chromium, it is derived from chromite ore (mainly FeCr).2O4) aluminothermic or silicothermic processes are used to form ferrochrome, an iron-chromium alloy. It is then used to form alloys such as ferrochrome, stainless steel. An alternative method is used to create pure chromium metal, which involves roasting and leaching the chromite to remove iron, followed by reduction with carbon and aluminum.
The trivalent chromium (Cr(III)) ion is recognized in the United States as an essential nutrient for human insulin, sugar, and lipid metabolism. In 2014, however, the European Food Safety Authority, speaking on behalf of the European Union, concluded that there was insufficient evidence to declare chromium necessary.
Hexavalent chromium, or Cr(VI), is toxic and carcinogenic, unlike chromium metal and Cr(III) ions, which are considered non-toxic. Chromium trioxide, used in industrial electroplating processes, is a "very high concern" substance according to the European Chemicals Agency (ECHA).
Cleaning up of abandoned chrome production sites is often necessary.
Physical Properties of Chromium Element
The electron configuration of chromium, the fourth transition metal in the periodic table, is [Ar] 3d5 4s1. The Aufbau principle was also broken for the first time in the periodic table by the ground state electron structure of this element. This is also true for other elements in the periodic table, such as copper, niobium, and molybdenum, and their electron configurations.
Because of their similar charge, electrons in the same orbital repel each other, causing this to happen. In previous elements, the energy released by reducing repulsion between electrons is insufficient to balance an electron's jump to the next higher energy level.
Since the 3d subshell is more compact than the 4s subshell, and there is a very small energy difference between the 3d and an upper 3s subshell in 4d transition metals, there is a smaller interelectron repulsion between the 4s electrons than between the 3d electrons. In this approach, the transition of electrons becomes possible and one or two electrons always pass into the 4s subshell. This increases the energy released.
Chromium's melting and boiling points and enthalpy of atomization are lower than its predecessor vanadium because chromium is the first element in the 3d series where 3d electrons begin to sink into the nucleus. As a result, they contribute less to metallic bonding. Unlike molybdenum(VI) and tungsten(VI) oxides, chromium(VI) is a strong oxidizer.
Pure chrome metal sample
Chromium is the third hardest element in terms of hardness, after carbon (diamond) and boron. Corundum can also scratch it, thanks to its Mohs hardness of 8,5, it can scratch samples of quartz and topaz. Unlike other metals such as copper, magnesium, and aluminum, chromium is very resistant to tarnishing, making it valuable as a metal that protects its outer layer from corrosion.
Compared to most of the transition metals, chromium has a relatively low melting point (1907 °C). It still has the second highest melting point of all Period 1910 elements, with vanadium surpassing it by 3°C at 4°C. However, its boiling point of 2671 °C is relatively lower and ranks fourth only among the 4th Period transition metals in terms of boiling point after copper, manganese and zinc. At 20 °C, chromium has an electrical resistance of 125 nanoohm-meters.
Compared to other transition metals, chromium has a high specular reflection. Chromium has a maximum infrared reflectance of approximately 425% at 72 m, this drops to a minimum of 750% at 62 m and rises once again to 4000% at 90 m. When chromium is used in polished stainless steel alloys, the specular reflection is still high compared to other alloys, although it decreases with the addition of additional metals. The visible spectrum is reflected by the polished stainless steel by 40-60%. The magnetic properties of chromium can be used to explain why it exhibits such a high proportion of reflected photon waves in general, 90% in the infrared in particular.
Chromium is the only elemental solid that exhibits antiferromagnetic order at and below ambient temperature, giving it special magnetic properties. Its magnetic arrangement becomes paramagnetic above 38°C. The magnetic properties of the body-centered cubic are disproportionate to the lattice periodicity, resulting in antiferromagnetic properties that cause the chromium atoms to momentarily ionize and bond with each other. This is due to the irregular but parallel nuclei of the cube and magnetic moments at its corners. From this point of view, chromium has a high infrared and visible light reflectivity due to its frequency dependent relative transmittance derived from Maxwell's equations and the antiferromagnetism of chromium.
Source: Wikipedia
📩 15/02/2023 12:56
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