# The Double Face of Metals: The Intriguing Case of Chromium
## 2 Chemical Form and Properties of Chromium  
Chromium is the 24 th element in the periodic table and its symbol is Cr. It was discovered in 1798 by [PERSON]. It belongs to transition metals and its average atomic weight is 52 g/mol. Its physicochemical properties are summarized in Table 1. Chromium is the first element of the sixth group; it is a steeply-grey, shiny, hard and brittle metal. It is the twenty-first most abundant element in the Earth's crust [25]. While it does not react with water, it may react with acids. Chromium may theoretically occur in oxidation states between \(-\)2 and +6, but the most common oxidation states are +3 and +6 [Cr(III) and Cr(VI)]. Cr\({}^{2+}\) is unstable and oxidizes to Cr\({}^{3+}\) when in contact with air. Elemental chromium (Cr(0)) is not present in nature in the Earth's crust and is physiologically inert. Cr(0) in its inert metallic form and at a concentration of about 11% is found almost exclusively as a constituent of iron alloys as stainless steel. The addition of 8% nickel to this alloy increases corrosion resistance [26]. Chromium owes its name to its many-colored compounds colors such as black, green, blue,violet, yellow, orange and red; thus, it has been used in paints and pigments. Common uses of chromium are summarized in Table 2. Chromium is both an essential trace element (as Cr3+) ion and an environmental toxicant (as Cr6+). The latter is considered a heavy metal. However, long time exposure to Cr(III) may lead to skin allergy and cancer [15,27]. In aqueous solutions, Cr(VI) may exist as chromate ion (CrO42-) or as dichromate or bichromate ion (Cr2O72-). Cr3+ in aqueous solutions is green, CrO42- confers a yellow color to water, while Cr2O72- is orange. Cr(VI) solutions are vigorous oxidizing agents in acidic conditions. Chromic acid (H2 CrO4) is utilized for cleaning glassware in chemistry laboratories by oxidizing organic residues. In anoxic conditions, a chromium trivalent form prevails, while the hexavalent form prevails in aerobic conditions. Chronate ions, especially potassium chromate (K2 CrO4), are used as indicators in the titration of chloride ions with silver nitrate in the Mohr method [28], while the potassium bichromate (K2 Cr2O7) titrimetric method is used to determine chemical oxygen demand (COD), that is the amount of oxygen needed to destroy all organic matter contained in water [29]. \begin{table}
\begin{tabular}{c c} \hline \hline Atomic number & 24 \\ \hline Atomic weight & 51.9961 u \\ \hline Atomic radius & 130 pm \\ \hline Electronic configuration & [Ar] 4s13d5 \\ \hline Melting point & 1907 °C \\ \hline Boiling point & 2672 °C \\ \hline Density at 20 °C & 7.18 g/cm3 \\ \hline Heat of fusion & 21 KJ/mol \\ \hline Heat of vaporization & 342 KJ/mol \\ \hline Pauling electronegativity number & 1.66 \\ \hline First ionization energy & 652.4 KJ/mol \\ \hline Second ionization energy & 1590.6 KJ/mol \\ \hline Third ionization energy & 2987 KJ/mol \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical and chemical properties of chromium. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**Form**} & **Uses** \\ \hline \hline \multirow{4}{*}{Cr(O)} & \multicolumn{2}{c}{Stainless steel production} \\ \cline{2-3}  & \multicolumn{2}{c}{Alloy production} \\ \cline{2-3}  & \multicolumn{2}{c}{Metal and alloy manufacturing} \\ \cline{2-3}  & \multicolumn{2}{c}{Metal and alloy manufacturing} \\ \cline{2-3}  & \multicolumn{2}{c}{Brick lining} \\ \cline{2-3}  & Cr(III) & Chrome plating and welding \\ \cline{2-3}  & \multicolumn{2}{c}{Leather tanning} \\ \cline{2-3}  & \multicolumn{2}{c}{Textiles} \\ \hline \hline \multirow{4}{*}{Cr(VI)} & \multicolumn{2}{c}{Chrome plating and welding} \\ \cline{2-3}  & \multicolumn{2}{c}{Copying machine toner} \\ \cline{2-3}  & \multicolumn{2}{c}{Chrome plating} \\ \cline{2-3}  & \multicolumn{2}{c}{Leather tanning} \\ \cline{2-3}  & \multicolumn{2}{c}{Textiles} \\ \cline{2-3}  & \multicolumn{2}{c}{Wood preservatives} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Uses of chromium.