Question

In diborane ($\text{B}_2\text{H}_6$), the number of bridge hydrogens is

• A. 2
• B. 4
• C. 6
• D. 0

Hint:

Remember that in $\text{B}_2\text{H}_6$, the two hydrogen atoms that are bonded to both boron atoms act as bridge hydrogens.

Answer 1

The Correct Option is A

Step 1: Picture diborane.
Diborane is $\text{B}_2\text{H}_6$. It has two boron atoms and six hydrogen atoms. Its bonding is special because boron has too few electrons.
Step 2: Two kinds of hydrogen.
In diborane the hydrogens come in two kinds. Some sit on only one boron, called terminal hydrogens. Some sit between the two borons, called bridge hydrogens.
Step 3: Count the terminal hydrogens.
Each boron carries two normal terminal hydrogens. With two borons that gives $2 \times 2 = 4$ terminal hydrogens.
Step 4: Find the bridge hydrogens.
Total hydrogens are six. Take away the four terminal ones to get the bridge ones.\[ 6 - 4 = 2 \]
Step 5: Why bridges form.
Boron does not have enough electrons for normal bonds. So two hydrogens link both borons at once using special three centre two electron bonds. These are the bridge hydrogens.
Step 6: Final choice.
So diborane has two bridge hydrogens.\[ \boxed{2} \]