According to the moleular orbital theory, all atomic orbitals combine to form molecular orbital by `LCAO` (linear combination of atomic orbitals) method When two atomic orbitals have additive (constructive) method When two atomic orbitals have additive (constructive) overlapping they form bonding molecular orbitals `(BMO)` which have lower energy than atomic orbitals whereas when atomic orbitals overlap subtractive higher energy antibonding molecular orbitals `(ABMO)` are formed Each `MO` occupies two electrons with opposite spin Distribution of electrons in `MO` follows Aufbau principle as well as Hund's rule `MO` theory can successfully explain the magnetic behaviour of molecules
Which of the following is/are not paramagnetic ? .

Options

A. `NO`
B. `B_(2)`
C. `CO`
D. `O_(2)`

Answer

To determine which of the given molecules are not paramagnetic, we need to analyze the number of unpaired electrons in each molecule based on the molecular orbital theory. Here's a step-by-step solution:

### Step 1: Understand the Definitions
- **Paramagnetic**: A species with one or more unpaired electrons.
- **Diamagnetic**: A species with all electrons paired (no unpaired electrons).

### Step 2: Identify the Total Number of Electrons
We will calculate the total number of electrons for each molecule given in the options.

### Step 3: Analyze Each Option
1. **NO (Nitric Oxide)**
- Nitrogen (N) has 7 electrons.
- Oxygen (O) has 8 electrons.
- Total = 7 + 8 = 15 electrons (odd).
- Since the total is odd, there is 1 unpaired electron.
- **Conclusion**: NO is paramagnetic.

2. **B2 (Diboron)**
- Boron (B) has 5 electrons.
- For two boron atoms: 5 + 5 = 10 electrons (even).
- For 10 electrons, there are 2 unpaired electrons.
- **Conclusion**: B2 is paramagnetic.

3. **CO (Carbon Monoxide)**
- Carbon (C) has 6 electrons.
- Oxygen (O) has 8 electrons.
- Total = 6 + 8 = 14 electrons (even).
- For 14 electrons, there are 0 unpaired electrons.
- **Conclusion**: CO is diamagnetic (not paramagnetic).

4. **O2 (Dioxygen)**
- Each oxygen has 8 electrons.
- For two oxygen atoms: 8 + 8 = 16 electrons (even).
- For 16 electrons, there are 2 unpaired electrons.
- **Conclusion**: O2 is paramagnetic.

### Step 4: Final Conclusion
From the analysis:
- **Paramagnetic Species**: NO, B2, O2
- **Not Paramagnetic (Diamagnetic)**: CO

Thus, the molecule that is **not paramagnetic** is **CO**.

---