Video: EC17-18-S2-Q06

EC17-18-S2-Q06

03:51

Video Transcript

Which of the following ion pairs both have the electronic configuration [Ar] 3d⁴? a) Mn²⁺ and Co²⁺. b) Fe³⁺ and Cr³⁺. c) Cr²⁺ and Mn³⁺. Or d) Fe²⁺ and Mn³⁺.

If you refer to your periodic table, you’ll see that all the elements in the answers are in the first transition series. Chromium, manganese, iron, and cobalt sit next to each other approximately in the middle of the table. The electronic configurations of all these elements all begin with Ar. Ar in square brackets indicates the electronic configuration of Argon 1s² 2s² 2p⁶ 3s² 3p⁶ and forms a convenient shorthand. And here are the reminders of the electronic configurations.

You’ll notice that in the electronic configuration of chromium, there’s only one electron in the 4s subshell. This is because one electron is promoted into the 3d subshell, making it more stable being half-full. In any case, in the formation of ions of these elements, it’s the 4s electrons which are lost first. We can now move on to figuring out the electronic configuration of the transition metal ions in each pair.

An atom of manganese with electronic configuration [Ar] 4s² 3d⁵ can lose two electrons to form Mn²⁺ with electronic configuration [Ar] 3d⁵. The question asks for the ion pair where both have the electronic configuration [Ar] 3d⁴. Since one of the ions in option a does not have the required electronic configuration, option a cannot be a correct answer.

However, I’m still going to show you the electronic configuration of cobalt ²⁺. An atom of cobalt will lose two electrons to form an ion of cobalt ²⁺ with electronic configuration [Ar] 3d⁷. As we’ve already demonstrated that option a is not the correct answer, we can move on to option b.

Iron three plus has the electronic configuration [Ar] 3d⁵ and chromium three plus has the electronic configuration [Ar] 3d³. Neither match the required electronic configuration. So, option b is not the correct answer.

What about option c? Chromium two plus has the electronic configuration [Ar] 3d⁴. And manganese three plus also has the electronic configuration [Ar] 3d⁴. Since option c has both ions with the electronic configuration [Ar] 3d⁴, it is the correct answer. But let’s have a look at option d just to be safe.

An atom of iron loses two electrons from the 4s subshell to form an ion of iron two with electronic configuration [Ar] 3d⁶. Whereas manganese three plus, as we’ve just seen, has the electronic configuration [Ar] 3d⁴. Since one of the ions in the ion pair d does not have the required electronic configuration, option d is not a correct answer. So, of the ion pairs given, the only one where both have the electronic configuration [Ar] 3d⁴ is option c, Cr²⁺ and Mn³⁺.

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