Q. hcn formal charge.

To find the formal charge in HCN, we first write the structure and then apply the formal charge formula to each atom.

Step 1: Write the usual Lewis structure of HCN

In HCN, hydrogen is single-bonded to carbon, and carbon is triple-bonded to nitrogen. A typical Lewis structure is:

\[
\text{H – C } \equiv \text{ N}
\]

This means:

• H forms one single bond to C
• C forms one single bond to H and three bonds to N (a total of 4 bonds around C)
• N forms a triple bond to C and has 1 lone pair

Step 2: Recall the formal charge formula

Use:

\[
\text{Formal charge} = V – (N_{\text{nonbonding}} + \tfrac{1}{2}N_{\text{bonding}})
\]

where:

• \(V\) is the number of valence electrons of the neutral free atom
• \(N_{\text{nonbonding}}\) is the number of electrons on the atom as lone pairs
• \(N_{\text{bonding}}\) is the total number of electrons shared in bonds involving that atom

Step 3: Formal charge on hydrogen (H)

Hydrogen has:

• Valence electrons \(V = 1\)
• No lone pairs, so \(N_{\text{nonbonding}} = 0\)
• One bond to carbon contributes \(N_{\text{bonding}} = 2\) electrons total around the H–C bond

So:

\[
\text{FC(H)} = 1 – \left(0 + \tfrac{1}{2}\cdot 2\right)
= 1 – 1
= 0
\]

Step 4: Formal charge on nitrogen (N)

Nitrogen has:

• Valence electrons \(V = 5\)
• One lone pair, so \(N_{\text{nonbonding}} = 2\)
• Triple bond to carbon means nitrogen is involved in three bonds. Total bonding electrons involving N is \(N_{\text{bonding}} = 6\)

So:

\[
\text{FC(N)} = 5 – \left(2 + \tfrac{1}{2}\cdot 6\right)
= 5 – \left(2 + 3\right)
= 0
\]

Step 5: Formal charge on carbon (C)

Carbon has:

• Valence electrons \(V = 4\)
• No lone pairs, so \(N_{\text{nonbonding}} = 0\)
• Carbon forms 4 bonds total (one to H and three to N), so its total bonding electrons are \(N_{\text{bonding}} = 8\)

So:

\[
\text{FC(C)} = 4 – \left(0 + \tfrac{1}{2}\cdot 8\right)
= 4 – 4
= 0
\]

Final answer: formal charges in HCN

• \(\text{FC(H)} = 0\)
• \(\text{FC(C)} = 0\)
• \(\text{FC(N)} = 0\)

Therefore, the molecule HCN has formal charge \(0\) on every atom.