# Question b2b26

Jan 6, 2016

$d$ orbitals are more stable when they are half filled or fully filled.

#### Explanation:

The copper element has an atomic number of $Z = 29$ which means that according to the Aufbau principle, the electron configuration should be:

""_29Cu:[Ar] 4s^(2)3d^(9)

However, it was found that copper's electron configuration as:

""_29Cu:[Ar] 4s^(1)3d^(10)

The explanation to this is that $d$ orbitals are more stable when they are half filled or fully filled.

There is a similar example which is Chromium $C r$ where the electron configuration is:

""_24Cr:[Ar] 4s^(1)3d^(5) instead of ""_24Cr:[Ar] 4s^(2)3d^(4)#.