Chemistry conceptual help??

Question

Exceptions: Cr and Cu

•   Cr: [Ar]4s13d5

•   Cu: [Ar]4s13d10 

whys the s orbital not filled?

 

Answer 1

Step 1:

1) Exception : .

We would expect chromium, the fourth transition metal to have the electronic configuration of .

However the electronic configuration is ,which shows a half filled 4s orbital and a half filled set of 3d orbital (one electron in each of five 3d orbitals).

Since the 4s and 3d orbitals are virtually degenerate in the chromium atom, we would expect the configuration rather than . 

Basically the chromium configuration occurs because the energies of the 3d and 4s orbitals are very similar to the first row transition elements.

So, the S-orbital is not completely filled & Chromium has an electronic configuration  . 

Step 2 :

(2) Exception : .

We would expect copper, the first row transition metal to have the electronic configuration of rather than .

For neutral transition metals, the 3d and 4s orbitals have very similar energies.

The energy of 3d orbitals in transition metal ions is significantly less than the 4s orbital.This means that the electrons remaining after the ion is formed occupy the 3d orbitals, since they are low in energy.

So the S-orbital is not completely filled &  Copper has an electronic configuration .

Solution :

(1) The S-orbital is not completely filled & Chromium has an electronic configuration  .

(2) The S-orbital is not completely filled &  Copper has an electronic configuration .