Video: EC17-18-S2-Q23



Video Transcript

Which of the following is true about ScCl₃? a) It is paramagnetic and coloured, b) it is paramagnetic and colourless, c) it is diamagnetic and coloured, or d) it is diamagnetic and colourless.

Let’s start by defining a couple of terms. Paramagnetic means that the compound has a net magnetic moment, while diamagnetic means the opposite, that there is no net magnetic moment. What this means for paramagnetic compounds is that they have a net spin. They have an uneven distribution of spin-up and spin-down electrons. This leads us to the first step of answering the question.

Chloride ions have the same electron configuration as argon. When electrons are populated in their respective subshells, we can see that there is an equal number of spin-up and spin-down electrons. Therefore, chloride ions have no net spin and are diamagnetic. We also know that chloride ions are colourless as they are in sodium chloride. Therefore, any paramagnetism or colour must arise from the scandium.

The electronic configuration of scandium can be determined from its position in the periodic table at the intersection of period four and group three. The period four indicates that the principal quantum number of a valence electron shell is four, while group three indicates there are a total of three electrons in the outer shell. But scandium trichloride does not contain scandium in the neutral state. It contains scandium(III). Scandium(III) has the same electronic configuration as argon and as chloride and is therefore diamagnetic. Since scandium chloride is definitely diamagnetic, options a) and b) can be dismissed.

Now what about colour? We already know that chloride does not have a colour. What about scandium 3+? Well, colour in d-block elements arises from d-d transitions. But we can see from the electronic configuration of scandium 3+ that there are no d-electrons to transition. Therefore, scandium 3+ is colourless. Therefore, of the statements given, the only one that is true about scandium trichloride is d). It is diamagnetic and colourless.

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