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In 2A3(b), we are asked to give the electron configuration for As3+. The configuration for As is normally [Ar]4s24p3. However, when As loses three electrons, the configuration is [Ar]3d104s2. I am comfortable with the idea that it loses 3 electrons from 4p3. However, I don't understand why the electron configuration is now [Ar]3d104s2 instead of just [Ar]4s2. Why include the 3d10 if you still haven't removed the 4s2 shell? Is it really valence in that case, especially when it is filled? On the flip side, why do we not write As as [Ar]3d104s24p3?
The electron configuration for As is [Ar]3d104s24p3. As3+ means that 3 electrons have been removed. When you're writing the electron configuration for As3+, you remove electrons from the valence shell. In this case, you remove electrons from the 4p orbital. Thus, you get [Ar]3d104s2. You cannot just remove 3d10 from the answer because you have not removed electrons from the 3d orbital yet.
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