Question
Why transition elements form a coloured ions?
| Configuration | Example | Colour |
| $3 d^0$ | $Sc ^{3+}$ | colourless |
| $3 d^0$ | $Ti ^{4+}$ | colourless |
| $3 d^{\prime}$ | $Ti ^{3+}$ | purple |
| $3 d^{\prime}$ | $V ^{4+}$ | blue |
| $3 d^2$ | $V ^{3+}$ | green |
| $3 d^3$ | $V ^{2+}$ | violet |
| $3 d^3$ | $Cr ^{3+}$ | violet |
| $3 d^4$ | $Mn ^{3+}$ | violet |
| $3 d^4$ | $Cr ^{2+}$ | blue |
| $3 d^5$ | $Mn ^{2+}$ | pink |
| $3 d^5$ | $Fe ^{3+}$ | yellow |
| $3 d^6$ | $Fe ^{2+}$ | green |
| $3 d^6 3 d^7$ | $Co ^{3+} Co ^{2+}$ | bluepink |
| $3 d^8$ | $Ni ^{2+}$ | green |
| $3 d^9$ | $Cu ^{2+}$ | blue |
| $3 d^{10}$ | $Zn ^{2+}$ | colourless |
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| t/s | 0 | 30 | 60 | 90 |
| [Ester]/mol L-1 | 0.55 | 0.31 | 0.17 | 0.085 |