The electronic configuration of valence shell of Cu is `3d^(10)4s^(1)` and not `3d^(9)4s^(2)`. How is this configuration explained?

Explanation

To explain why the electronic configuration of the valence shell of copper (Cu) is `3d^(10)4s^(1)` instead of `3d^(9)4s^(2)`, we can follow these steps:

### Step 1: Determine the Atomic Number of Copper
Copper has an atomic number of 29, which means it has 29 electrons in its neutral state.

### Step 2: Write the Ground State Electronic Configuration
The ground state electronic configuration of copper can be derived from the electron filling order. The configuration is:
- Argon (which accounts for the first 18 electrons): `1s^2 2s^2 2p^6 3s^2 3p^6`
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