Fix the mistake: Inorganic Chemistry (Transition Elements and Complexes)

Characteristics of Transition Elements

Groups 3–11 (or, including Group 12, 3–12) of the periodic table are called transition elements; they are groups of metals with electrons in d orbitals. They have many characteristics different from those of typical elements.

Basic Knowledge

Characteristics of transition elements: ① all are metals, ② high density (high melting points and high density), ③ can have multiple oxidation states, ④ many form colored ions, ⑤ readily form complex ions, ⑥ many show catalytic activity, and ⑦ some show ferromagnetism (Fe, Co, and Ni).
Main transition elements and oxidation states: Cr (+3, +6), Mn (+2, +4, +6, +7), Fe (+2, +3), Cu (+1, +2), Ag (+1), and Zn (+2; although it behaves like a typical element because it belongs to Group 12 and has d10).
Examples of colors: Cu2+ (blue), Fe2+ (pale green), Fe3+ (yellow-brown), Cr3+ (green), Cr2O72- (orange), CrO42- (yellow), and MnO4- (red-purple). The colors are caused by d–d transitions or charge-transfer transitions.

Important Terms and Formulas
Transition elements (metals with electrons in d orbitals, Groups 3–11)
d–d transition (electron transition between d orbitals; color produced by absorbing visible light)
Multiple oxidation states (Fe:+2/+3, Cu:+1/+2, Mn:+2 to +7)
Unpaired electrons (the cause of magnetism; Fe3O4 is ferromagnetic)
Catalytic activity (many examples, including Fe, Pt, V2O5, Ni, and Pd)
Alloys (solid solutions of multiple metals, such as steel, stainless steel, and solder)

Further Study (Principles and Applications)

The catalytic activity of transition metals is fundamental to modern chemical industry. Fe (Haber–Bosch process), Pt–Rh (Ostwald process), V2O5 (contact process), Ni (hydrogenation), and Pd (cross-coupling and Nobel Prize-winning research) are examples. The flexibility of d-orbital electrons stabilizes reaction intermediates and lowers activation energy.
Alloys are also important in applications: stainless steel (Fe–Cr–Ni), duralumin (Al–Cu–Mg–Mn), bronze (Cu–Sn), brass (Cu–Zn), solder (Sn–Pb → currently lead-free), shape-memory alloy (Ni–Ti), and neodymium permanent magnets (Nd–Fe–B).
Regarding magnetism, Fe, Co, and Ni are ferromagnetic, while Mn and Cr are antiferromagnetic. Rare earths are actually lanthanoids rather than transition elements, but they are similarly important in magnets, phosphors, and catalysts.

Key Points
  • All transition elements are metals
  • Multiple oxidation states are possible because d electrons can be removed or added
  • Color is caused by d–d transitions or charge-transfer transitions
  • Catalytic activity and magnetism are characteristic
  • Ferromagnetic elements: Fe, Co, and Ni
  • Alloys improve mechanical properties
  • Zn (Group 12) is often treated as a typical element

Points to Note (Common Confusions and Frequent Errors)

① The range of transition elements differs among textbooks: Groups 3–11 or Groups 3–12 (depending on whether Zn is included). ② The color difference between Fe2+ (pale green) and Fe3+ (yellow-brown) is frequently tested. ③ Chromate/dichromate ions are in equilibrium depending on the pH: 2CrO42- + 2H+ ⇄ Cr2O72- + H2O (yellow ↔ orange).

Practice

  1. Explain from their electron configurations why transition elements can have multiple oxidation states.
  2. State the colors of aqueous Fe2+ and Fe3+.
  3. Write the equilibrium equation between chromate and dichromate ions and explain how it shifts with pH.

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