3d Series Elements
Trends in Properties from Scandium (Z=21) to Zinc (Z=30).
The 3d series (First Transition Series) involves the filling of the 3d subshell. These elements exhibit characteristic properties due to the presence of unpaired d-electrons.
1. Electronic Configuration
General Configuration: $[Ar] 3d^{1-10} 4s^{1-2}$.
| Element | Symbol | Z | Configuration | Note |
|---|---|---|---|---|
| Scandium | Sc | 21 | $3d^1 4s^2$ | - |
| Titanium | Ti | 22 | $3d^2 4s^2$ | - |
| Vanadium | V | 23 | $3d^3 4s^2$ | - |
| Chromium | Cr | 24 | $3d^5 4s^1$ | Half-filled Stability |
| Manganese | Mn | 25 | $3d^5 4s^2$ | Half-filled d-subshell |
| Iron | Fe | 26 | $3d^6 4s^2$ | - |
| Cobalt | Co | 27 | $3d^7 4s^2$ | - |
| Nickel | Ni | 28 | $3d^8 4s^2$ | - |
| Copper | Cu | 29 | $3d^{10} 4s^1$ | Fully-filled Stability |
| Zinc | Zn | 30 | $3d^{10} 4s^2$ | Not a transition metal |
2. Atomic Radii Trends
The Curve
The variation in atomic radii across the 3d series is not uniform.
- Sc to Cr (Decrease): As nuclear charge increases, the shielding by d-electrons is not effective enough to counter it. Radius decreases.
- Mn to Ni (Constant): The increasing nuclear charge is balanced by the screening effect of d-electrons. Radius remains almost constant.
- Cu to Zn (Increase): At the end of the series, electron-electron repulsion in the fully filled d-orbitals dominates over nuclear attraction. Radius increases.
3. Oxidation States
Variable Valency
Transition metals show variable oxidation states because the energy difference between $(n-1)d$ and $ns$ orbitals is very small.
- Common State: +2 (due to loss of $4s^2$). Exception: Sc usually shows +3.
- Highest State: Increases from Sc to Mn, then decreases. The maximum oxidation state is given by the sum of $ns$ and unpaired $(n-1)d$ electrons.
- Manganese: Shows the widest range (+2 to +7).
- Zinc: Only shows +2 (stable $3d^{10}$ configuration).
4. Standard Electrode Potentials ($E^\circ_{M^{2+}/M}$)
Irregular Trend
Generally, the values are negative (metals are reducing agents). However, the trend is irregular due to variations in Sublimation Enthalpy, Ionization Enthalpy ($IE_1 + IE_2$), and Hydration Enthalpy.
5. Ionization Enthalpy (IE)
Generally increases from left to right due to increased nuclear charge.
- The increase is not as steep as in representative elements due to shielding by d-electrons.
- Exceptions: The $IE_1$ of Zn is very high (stable $d^{10}s^2$). The $IE_2$ of Cr and Cu are unusually high because removal of the second electron disrupts stable $d^5$ and $d^{10}$ configurations.
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