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
Bond strength increases when

Options

A. bond order increases
B. bond length increases
C. antibonding electrons increases
D. bond angle increases

Explanation

To solve the question regarding the conditions under which bond strength increases, we will analyze the relationship between bond order, bond length, and other factors affecting bond strength.

### Step-by-Step Solution:

1. **Understanding Bond Strength**:
- Bond strength refers to the energy required to break a bond between two atoms. It is influenced by the type of bond (single, double, triple) and the atomic orbitals involved.

2. **Bond Order**:
- Bond order is defined as the number of bonding electrons minus the number of antibonding electrons, divided by two.
- A higher bond order indicates a stronger bond. For example, a triple bond has a bond order of 3, a double bond has a bond order of 2, and a single bond has a bond order of 1.

3. **Relationship Between Bond Order and Bond Strength**:
- As bond order increases, bond strength also increases. This is because a higher bond order means more electrons are involved in bonding, which leads to a stronger attraction between the nuclei of the bonded atoms.

4. **Bond Length**:
- Bond length is inversely proportional to bond strength. This means that as bond strength increases (due to higher bond order), bond length decreases. A shorter bond length indicates a stronger bond.

5. **Effect of Antibonding Electrons**:
- The presence of antibonding electrons (in molecular orbitals) can weaken the bond. Therefore, an increase in antibonding electrons generally leads to a decrease in bond strength.

6. **Bond Angle**:
- Bond angle can also influence bond strength. A larger bond angle can indicate less steric strain and a more stable molecular structure, which can correlate with stronger bonds.

7. **Evaluating Options**:
- **Option A**: Bond strength increases when bond order increases. (True)
- **Option B**: Bond strength increases when bond length increases. (False, as bond length and bond strength are inversely related)
- **Option C**: Bond strength increases when antibonding electrons increase. (False, as more antibonding electrons weaken the bond)
- **Option D**: Bond strength increases when bond angle increases. (True, as larger bond angles can indicate less strain and more stability)

### Conclusion:
The correct options that indicate conditions under which bond strength increases are **A** and **D**.