Question

Explain why chromium has only one electron in its 4s sub–shell?

Answer 1

One of the 4s electrons is moved to the 3d subshell because the element is more stable and has lower energy when the d is either half full or completely full. Copper does the same thing to have 10 electrons instead of 9 in the 3d subshell.

This makes the configuration of Cr: [Ar]4s^1 3d^5
And Cu is therefore: [Ar]4s^1 3d^10

NB. Where [Ar] is the noble gas configuration up to that of Argon and ^ represents superscript, to avoid confusion there.

Answer 2

Chromium element chemical symbol is “Cr”.

Chromium atomic number is 24

$Electronic\quad configuration\quad =\quad [Ar]{ 3d }^{ 5 }{ 4s }^{ 1 }$

It's more stable because the subshells 4s and 3d are close to energy.  

Therefore, the energy increases due to the filling of a subshell of 4s (having two opposite spin electrons are placed in one orbital) is “greater” than the increase in energy due to the slightly higher unfilled 3d orbital energy in the electron.