CBSE 2025, Set 3

Question

In which of the following groups are both ions coloured in aqueous solution?

I. Cu$^+$  II. Ti$^{4+}$  III. Co$^{2+}$  IV. Fe$^{2+}$

[Atomic number : Cu = 29, Ti = 22, Co = 27, Fe = 26] A). I and II B). II and III C). III and IV D). I and IV

Edvaya · Accepted Answer

The correct answer is III and IV. Coloured ions require **partially filled d-orbitals** (for d–d electronic transitions): - **Cu$^+$ (I):** Cu = [Ar]3d$^{10}$4s$^1$ $\rightarrow$ Cu$^+$ = [Ar]3d$^{10}$ $\rightarrow$ **completely filled d** $\rightarrow$ **colourless**. - **Ti$^{4+}$ (II):** Ti = [Ar]3d$^2$4s$^2$ $\rightarrow$ Ti$^{4+}$ = [Ar] (no d electrons) $\rightarrow$ d$^0$ $\rightarrow$ **colourless**. - **Co$^{2+}$ (III):** Co = [Ar]3d$^7$4s$^2$ $\rightarrow$ Co$^{2+}$ = [Ar]3d$^7$ $\rightarrow$ **partially filled** $\rightarrow$ **coloured** (pink/blue). - **Fe$^{2+}$ (IV):** Fe = [Ar]3d$^6$4s$^2$ $\rightarrow$ Fe$^{2+}$ = [Ar]3d$^6$ $\rightarrow$ **partially filled** $\rightarrow$ **coloured** (pale green). **Answer: III and IV (Co$^{2+}$ and Fe$^{2+}$)**

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Chemistry > Inorganic Chemistry > D And F Block Elements

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