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The 3d orbital comes before the 4s orbital because the 3d state has lower energy - making it more difficult to remove. When looking at which electrons an atom will give away, it is most likely to give away the "s" electrons because they have higher energy and therefore, are easier to remove - typically the two "s" electrons travel as a pair. I was taught to write 4s before 3d in high school, however conceptually this answer avoids confusion and I assume Dr. Lavelle prefers it.
The fact that 3d comes before 4s can be confusing because the d-block on the periodic table doesn't begin until the 4th period, so try to think about it in the context of the angular momentum quantum numbers instead of the literal rows on the periodic table. If n=4 then l can equal 3 which corresponds to the d-orbital
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