Question

Why Cu has the ground state electronic configuration as $\left[Ar\right]4s^{1}3d^{10}$ instead of having $\left[Ar\right]4s^{2}3d^9$ ?

Answer 1

There are two main exceptions to electron configuration: chromium and copper. In these cases, a completely full or half full $d$ sub-level is more stable than a partially filled $d$ sub-level, so an electron from the $4s$ orbital is excited and rises to a $3d$ orbital.
Using the Aufbau principle, you would write the following electron configurations
$Cr=\left[Ar\right]4s^{2}3d^4$
$Cu=\left[Ar\right]4s^{2}3d^9$
The actual electron configurations are:
$Cr=\left[Ar\right]4s^{1}3d^5$
$Cu=\left[Ar\right]4s^{1}3d^{10}$
To understand why this occurs, it is important to realize that...
1. Completely filled sublevels are more stable than partially filled sublevels.
2. A sublevel which is exactly half filled is more stable than a partially filled sublevel which is not half full.
3. Electrons move in the places of the lowest energy state which is the most stable state.
In both examples, an electron moves from the $4s$ sublevel to produce a $1/2$ full $3d\left(Cr\right)$ or completely filled $3d(Cu$). This gives the atom greater stability so the change is favorable.