Why do boron halides exist as monomeric species?

Explanation

### Step-by-Step Solution

1. **Understanding Boron Halides**: Boron halides, such as BF3, BCl3, BBr3, etc., are compounds formed between boron and halogen elements. They are generally represented as BX3, where X is a halogen.

2. **Hybridization of Boron**: In boron halides, the boron atom undergoes sp² hybridization. This means that one s orbital and two p orbitals of boron mix to form three equivalent sp² hybrid orbitals, which are used to bond with the three halogen atoms.

3. **Valence Electrons**: Boron has three valence electrons and, in forming BX3, it uses all three to bond with the halogens. However, this leaves boron with only six electrons in its outer shell, which is less than the octet rule.

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