The number of unpaired electrons in `O_2` is ……

Answer

To determine the number of unpaired electrons in the O₂ molecule, we will follow these steps:

### Step 1: Determine the total number of electrons
Oxygen (O) has an atomic number of 8, meaning each oxygen atom has 8 electrons. Since O₂ consists of two oxygen atoms, the total number of electrons in O₂ is:
\[ 8 \text{ (from one O)} \times 2 = 16 \text{ electrons} \]

### Step 2: Write the molecular orbital configuration
The molecular orbital (MO) theory describes how atomic orbitals combine to form molecular orbitals. For O₂, the order of filling the molecular orbitals is as follows:
1. σ(1s)
2. σ*(1s)
3. σ(2s)
4. σ*(2s)
5. σ(2p_z)
6. π(2p_x) and π(2p_y) (these orbitals are degenerate)
7. π*(2p_x) and π*(2p_y) (these orbitals are also degenerate)
8. σ*(2p_z)

### Step 3: Fill the molecular orbitals with electrons
Now, we will fill these molecular orbitals with the 16 electrons according to the Aufbau principle, Pauli exclusion principle, and Hund's rule:
- σ(1s): 2 electrons
- σ*(1s): 2 electrons
- σ(2s): 2 electrons
- σ*(2s): 2 electrons
- σ(2p_z): 2 electrons
- π(2p_x): 1 electron
- π(2p_y): 1 electron

At this point, we have filled:
- 2 + 2 + 2 + 2 + 2 + 1 + 1 = 12 electrons in bonding orbitals
- The remaining 4 electrons will fill the antibonding orbitals.

### Step 4: Identify unpaired electrons
From the filling:
- The π(2p_x) and π(2p_y) orbitals each have 1 electron, which means they are unpaired.
- The σ(2p_z) orbital is fully filled with 2 electrons (paired).

Thus, the total number of unpaired electrons in O₂ is:
\[ 1 \text{ (from π(2p_x))} + 1 \text{ (from π(2p_y))} = 2 \text{ unpaired electrons} \]

### Conclusion
The number of unpaired electrons in O₂ is **2**.

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