In other words, let's look at the following important facts. So four S that energy level is at a lower energy than three D, and five S is at a lower energy level than four D.
And this holds for six S and 5D as well. Now, these things you just have to simply remember. Now, if we look at the electron configuration for Bromine, which has 35 35 protons and 35 electrons, we see the following electron configuration. Now, this guy makes sense because one S comes before two S and two S comes before two P and two P comes before three S, and three S comes before three P. In other words, this goes from lowest energy level to highest energy level. And since we just said that four S is at a lower energy level than 3D, it would make sense to place the four S two before the this would be the correct election configuration if you're actually ordering from lowest energy level to highest energy level.