in the periodic table, why does 4s come first before 3d in the electron configuration?

Question

teacher mentioned caroll lehrman principle

Answer 1

It's all about which electron shell is a lower energy state. 4s is lower energy than 3d. These energy levels are kind of confusing since they seem to defy logic and actually are inconsistent. But what it comes down to, is an extra electron always finds the orbital of lowest energy, and we know that they occupy the higher level s-orbital before a level-1 d orbital is filled.

Answer 2

In the hydrogen atom, 3s, p, d are all the same energy and below 4s. So what is happening in many-electron atoms?

Actually, the 4s spend some of its time (if you prefer more technical language, has some probability density) close in to the nucleus, so it is incompletely screened from the nucleus by the other electrons. The balance between 4s and 3d is pretty delicate, and in cations (such as transition metal ions) 3d is below 4s.

Examples: Fe atom, 4s23d6; Ni2+ 3d8

Does that help?

Answer 3

It always goes with the lower energy first.