Orbital Order

[ngarcia]

In today's lecture, Professor Lavelle brought up how 3d comes before 4s, contrary to what most of us learned in high school. Can someone explain why this is the case?

[Andrew F 2L]

I think Lavelle meant that the 3d orbital is a higher energy state than the 4s state so the electrons fill it out first and then start to fill the 4s state.

[Julia Mazzucato 4D]

Yes, Andrew F is right. Even though the 4s orbital is chronologically before the 3d orbital on the periodic table, an orbital's energy level and thus its order of being filled is determined by its first quantum number n, not its quantum number l (which indicates if the orbital is s,p,d,f). Basically the n value, in this case, takes precedence over the l value. It was taught the other way in high school mostly for the sake of not being super confusing, since quantum mechanics wasn't really in the curriculum of what we had to understand back then I suppose.