## 4s and 3d

Baoying Li 1B
Posts: 113
Joined: Sat Aug 17, 2019 12:18 am

### 4s and 3d

Hi, can anyone explain why does 4s come before 3d since the energy level n=3 is lower than n=4? I know Professor Lavelle has explained this, but I am still confused.

Ronak Naik
Posts: 103
Joined: Sat Jul 20, 2019 12:15 am

### Re: 4s and 3d

Hi, when writing the electron configuration 3d actually comes first before 4s since 3d electrons are at a lower electron shell level.

Benjamin Feng 1B
Posts: 102
Joined: Sat Sep 07, 2019 12:19 am

### Re: 4s and 3d

For n<20 4s comes before 3d, as when the 3d orbital is empty, the energy of the orbital is higher. The reasoning of why the 2 switch when n>20 is becuase protons are added to the nucleus and electrons surrounding the nucleus. The various attractions and repulsions in the atoms cause the energies to shift around and they are not always constant.

Just know that everything up to Calcium will have 4s first and everything after has 3d first.

Emma Popescu 1L
Posts: 105
Joined: Wed Sep 11, 2019 12:16 am

### Re: 4s and 3d

The 4s is filled first because it is at a lower energy than 3d. However, Dr. Lavelle wants us to put them from lowest to highest energy level (3d and then 4s) because 4s is ionized first.

Jocelyn Thorp 1A
Posts: 103
Joined: Wed Sep 18, 2019 12:20 am

### Re: 4s and 3d

Emma Popescu 3D wrote:The 4s is filled first because it is at a lower energy than 3d. However, Dr. Lavelle wants us to put them from lowest to highest energy level (3d and then 4s) because 4s is ionized first.

ok so, 4s is filled first, and 3d is written first? thanks.

Emily Chirila 2E
Posts: 112
Joined: Sat Jul 20, 2019 12:16 am

### Re: 4s and 3d

You should fill the shells in the order they appear on the PT, but write them in order of values of n. So you would fill 4s before 3d, but when you write the configuration, say for titanium, you would write [Ar] 3d2 4s2.