What is Cr? Important knowledge about Cr.

1. Definition of Cr
   Chromium, also known as Crôm (in English is Chromium), symbolized as Cr, is a chemical element in the periodic table with an atomic number of 24 and an atomic mass of 51.9961. Chromium is a hard, ductile metal with good tensile strength. Chromium can exist in various states of oxidation, from Cr(II) to Cr(VI).
   The common ion structure of Chromium is Cr₃+ or Cr₂+.

2. Properties of Cr

2.1 Physical properties of Cr
At standard state, Chromium is a hard metal, with a silver-gray color and no specific smell. It is insoluble in water and slightly soluble in acid. Its pH is neutral.

2.2 Chemical properties of Cr
Chromium reacts with oxygen to form a Cr₂O₃ coating that helps prevent corrosion. Chromium can also react with strong acids such as HCl to produce CrCl₃ and H₂ gas.

3. Common chemical equations of Cr
   Reaction of Cr with O₂: 4Cr + 3O₂ -> 2Cr₂O₃.
   Reaction of Cr with HCl: 2Cr + 6HCl -> 2CrCl₃ + 3H₂.

4. Preparation of Cr
   4.1 Laboratory preparation of Cr
   Chromium is typically prepared from chromite ore (FeCr₂O₄) by roasting the ore with excess Na₂CO₃ at high temperatures, then treated with acid to obtain Cr₂O₃, which is then reduced with charcoal to obtain Cr.

4.2 Industrial preparation of Cr
In industry, Chromium is prepared through a 2-step reduction process from chromite ore: the ore is roasted with excess Na₂CO₃ at high temperatures, then treated with acid to obtain Cr₂O₃, which is then reduced with Al to obtain Cr.