Video: EC17-18-S1-Q22

Which of the following compounds is not either colored or paramagnetic? [A] FeCl₃ [B] CuCl₂ [C] ScCl₃ [D] CoCl₂

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Video Transcript

Which of the following compounds is not either colored or paramagnetic?

In order to be colored, a compound must have an energy gap in the visible range and electrons capable of being promoted to absorb light, while being paramagnetic means that the compound has to have unpaired electrons. The cations in each of these compounds are all derived from d-block elements, iron, copper, scandium, and cobalt. All these elements are in period four of the periodic table. Scandium is in the far left of the d-block. Iron and cobalt are roughly in the middle. And copper is towards the right. We know that the chloride ion must be colorless as it is in sodium chloride. Therefore, any color for these compounds must arise from the d-block elements.

Any color of a d-block element arises due to electron transitions in the partially filled d-subshells. Therefore, we should look at the electron configurations of the d-block elements in each of these compounds, to see which ones have partially filled d-subshells. Iron, in iron three chloride, has the electronic configuration of an argon core 3d5. We get this electron configuration by starting with the electron configuration of the pure element and taking away three electrons, first from the 4s orbital and then from the 3d subshells.

Completing the electronic configurations of the other d-block cations, we get copper two with [Ar] 3d⁹, scandium three with [Ar] 3d⁰, and cobalt two [Ar] 3d⁷. Of these four compounds, scandium three chloride is the only one where the d-block metal has no partially filled d-subshells. Therefore, scandium three chloride is not colored. The other three compounds have d-metal cations with odd numbers of electrons. Since electrons can either be spin up or spin down, having an odd number electrons means that there must be a net spin. Therefore, these three compounds are paramagnetic. Scandium three chloride, on the other hand, has no d-electrons whatsoever and therefore cannot have a net spin and therefore cannot be paramagnetic.

Therefore, of the four compounds given, the only one that is not either colored or paramagnetic is scandium three chloride.

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