How many unpaired electrons are in a copper atom?

To determine the number of unpaired electrons in a copper atom, we need to look at its electron configuration and understand how electrons are arranged in atomic orbitals.

### Step 1: Determine the Atomic Number

Copper (Cu) has an atomic number of 29, meaning it has 29 electrons when it is neutral.

### Step 2: Write the Electron Configuration

The electron configuration for copper can be written as follows:

1. Fill the 1s orbital first:
- 1s²
2. Next, fill the 2s and 2p orbitals:
- 2s² 2p⁶
3. Continue to fill the 3s and 3p orbitals:
- 3s² 3p⁶
4. The 4s orbital is filled before the 3d orbitals:
- 4s²
5. Then, fill the 3d orbitals. Copper has an exceptional configuration where one electron is removed from the 4s orbital to half-fill the 3d orbitals:
- 3d¹⁰

Thus, the complete electron configuration for copper is:

\[
\text{Cu: } [\text{Ar}] \, 4s^1 \, 3d^{10}
\]

### Step 3: Identify Unpaired Electrons

Now, let’s look at the electron configuration:

- The 4s orbital has **1 electron** (unpaired).
- The 3d orbital has **10 electrons**. According to Hund’s rule, electrons fill each orbital singly before pairing. However, since the 3d subshell has 5 orbitals (3d₁, 3d₂, 3d₃, 3d₄, 3d₅), all five orbitals are filled completely, meaning they are paired up and contribute **0 unpaired electrons**.

### Conclusion

In a copper atom, there is **1 unpaired electron** in the 4s orbital. Therefore, the total number of unpaired electrons in a copper atom is **1**.