Characteristics of Transition Elements

A transition element is an element that has a partially filled 'd' orbital. These elements have at least one unpaired electron in the outermost sub-energy level 'd'. Transition elements are characterized by incompletely filled d-orbitals. They are called transition elements because they exhibit intermediate properties between the 's' block and 'p' block elements. Transition elements are typically d-block elements, but not all d-block elements are considered transition elements. Most of the transition elements are metals, and are referred to as transition metals.

There are three series of transition metals:

(i) Transition metals of the first series.

(ii) Transition metals of the second series or Lanthanide metals.

(iii) Transition metals of the third series or Actinide metals (the strongest metals).

Transition metals of the first series

These metals have half-filled 3d-orbitals and at least one unpaired electron in the 3d sub-energy level. The transition metals of the first series are listed below:

Atomic Number	Element	Symbol	Electronic Structure
21	Scandium	Sc	4s² 3d¹
22	Titanium	Ti	4s² 3d²
23	Vanadium	V	4s² 3d³
24	Chromium	Cr	4s¹ 3d⁵
25	Manganese	Mn	4s² 3d⁵
26	Iron	Fe	4s² 3d⁶
27	Cobalt	Co	4s² 3d⁷
28	Nickel	Ni	4s² 3d⁸
29	Copper	Cu	4s¹ 3d¹⁰
30	Zinc	Zn	4s² 3d¹⁰

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General properties of transition metals

Transition metals exhibit the following general properties:

1. Form colored compounds.
2. Are paramagnetic substances.
3. Have variable oxidation states.
4. Form complex compounds.
5. Have catalytic properties.

Color formation by transition metals

Transition metals form colored compounds at room temperature or when in ionic or combined states. Non-transition metals do not form colors at room temperature, but they do when heated during a flame test. The energy from the flame excites the electrons, causing them to jump to a higher energy level. To return to a stable state, the electrons fall back to the ground state, emitting radiant energy which is detected as color.

Transition metals exhibit color due to two main theories:

• 4s – 3d electron transition theory
• d-d electron transition theory (crystal field theory)

4s – 3d electron transition theory

According to this theory, color formation is the result of the movement of electrons between the 4s and 3d sub-energy levels. The energy difference between these sub-levels is small enough that radiant energy can excite the electrons, causing them to jump from the 4s level to the 3d level. The unstable excited electrons fall back to the 4s orbital, emitting heat energy as radiation. This radiation is detected as color.

d-d electron transition theory (crystal field theory)

Color formation can also be explained by crystal field theory, which suggests that for transition metals to exhibit color, two conditions must be met:

1. There must be at least one unpaired electron in the 3d orbital.
2. There must be the presence of ligands.

In isolated transition atoms, all five 3d-orbitals are degenerate, meaning they have equal energy. In the presence of ligands, the d-orbitals split into two sets with different energy levels: double degenerate and treble degenerate orbitals. The repulsion force from ligands causes this splitting. The energy difference (E) between these two degenerates is small enough that normal radiant energy can excite electrons, causing them to jump from the treble degenerates to the double degenerates. This transition results in the emission of radiation, which is detected as color.

The intensity of the color depends on the splitting of the d-orbitals and the strength of the ligand's electric field:

• Ligands of high spin: These ligands exert weak electric fields, causing a small separation between the d-orbitals. The resulting color is light or faint.
• Ligands of low spin: These ligands exert strong electric fields, causing a large separation between the d-orbitals. The resulting color is deep.

Ligands of low spin include nitrogen-containing compounds e.g., NH₃, CN⁻, CN etc.