When elements in the 4th period are considered, the newly added electrons are filled in d subshell by adding one electron each from Sc to Zn. But the electronic configuration of chromium and copper are different from the expected configurations as given below:
| Element | Expected electronic configuration | Real electronic configuration |
|---|---|---|
| Cr | 1S2 2S2 2P6 3S2 3P6 3d4 4S2 | 1S2 2S2 2P6 3S2 3P6 3d5 4S1 |
| Cu | 1S2 2S2 2P6 3S2 3P6 3d9 4S2 | 1S2 2S2 2P6 3S2 3P6 3d10 4S1 |
The configurations with half filled d subshell or completely filled d subshell show greater stability
Similarly in f subshell f7,f14 arrangements show greater stability.