Which ion is colourless? - DASH

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Question

Which ion is colourless?

Cr⁴⁺

Se³⁺

Ti³⁺

V³⁺

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Well-Structured Explanation 1. Background on Color in Ions Many ions of transition metals appear colored primarily due to the presence of unpaired electrons in their partially filled d-orbitals. These unpaired electrons can transition between different d-orbital energy levels (often referred to as d–d transitions) when light is absorbed, leading to characteristic colors. 2. Electron Configurations of the Given Ions Cr4+: Chromium normally is $[Ar]\,3d^5\,4s^1$. Losing four electrons results in $[Ar]\,3d^2$. With two unpaired d-electrons, it can show color. Ti3+: Titanium normally is $[Ar]\,3d^2\,4s^2$. Losing three electrons results in $[Ar]\,3d^1$. With one unpaired d-electron, it shows color. V3+: Vanadium normally is $[Ar]\,3d^3\,4s^2$. Losing three electrons results in $[Ar]\,3d^2$. With two unpaired d-electrons, it exhibits color. Se3+: Selenium is a p-block element ($[Ar]\,3d^{10}\,4s^2\,4p^4$ in its neutral state). Forming a $3+$ ion typically removes electrons from the 4p and possibly 4s level, leaving no unpaired d-electrons. It thus cannot undergo d–d transitions. 3. Reason for Colorlessness of Se3+ Since Selenium is not a transition metal, it does not have partially filled d-orbitals in the $3+$ oxidation state. Without unpaired electrons in d-orbitals, there are no d–d transitions responsible for visible coloration. Therefore, Se3+ is colorless. 4. Conclusion Among the options given, Se3+ has no unpaired d-electrons and hence does not show any characteristic d–d transition. Consequently, it remains colorless, while the transition metal ions with unpaired d-electrons (Cr4+, Ti3+, and V3+) exhibit colors.

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