stays the same.

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Take a look at Fig. 6.28 on page 205. I worry just a bit, that the sentence for this question, which was taken directly from the book, is incorrect. You can see that if you start in the 3d column, you can go quite a ways without changing the 3, but eventually you get to 4p, and to me, that is a change in principal quantum number, (from n=3 to n=4.)