## 4s orbitals and 3d orbitals

JamieVu_2C
Posts: 108
Joined: Thu Jul 25, 2019 12:16 am

### 4s orbitals and 3d orbitals

In the book, it says "Argon completes the third period. From Fig. 1E.1, you can see that the energy of the 4s-orbital is slightly lower than that of
the 3d-orbitals. As a result, instead of electrons entering the 3d-orbitals, the fourth period now begins by filling the 4s-orbitals..."

How is 4s lower in energy than 3d, and if 4s is lower in energy, why is 3d written before 4s in the electron configurations? Why would the valence electrons be removed from 4s first before going to the 3d orbitals if 3d is higher in energy?

Justin Vayakone 1C
Posts: 110
Joined: Sat Sep 07, 2019 12:19 am

### Re: 4s orbitals and 3d orbitals

When elements have $z\leq 20$, then the 4s orbital is less in energy than 3d meaning 4s is written first. In other words, elements up to Calcium will have the 4s state come before 3d. But for elements after calcium in the periodic table, the 4s state is actually higher in energy than 3d. For example if we are writing the electronic configuration of Scandium, it would be: $[\mathrm{Ar}]3d^14s^2$. Now referring back to the textbook, since Argon is before Calcium, the 4s orbital comes first.

Jacey Yang 1F
Posts: 101
Joined: Fri Aug 09, 2019 12:17 am

### Re: 4s orbitals and 3d orbitals

As Dr. Lavelle explained in lecture, the 4s state is higher in energy than 3d once the 4s state becomes occupied and the electron enters the 3d state.