Chart illustrating the total valence electrons and Lewis structures for common molecules.

Actually, there is an error in the Lewis structure shown for HCN in the table.

Let’s analyze and correct it step by step.

---

## ✔ Step 1: Verify Total Valence Electrons for HCN

- Hydrogen (H): 1 valence electron
- Carbon (C): 4 valence electrons
- Nitrogen (N): 5 valence electrons

Total = 1 + 4 + 5 = 10 valence electrons → ✔️ Correct as shown.

---

## ✔ Step 2: Determine Correct Lewis Structure for HCN

HCN is a linear molecule: H — C ≡ N

- Hydrogen can only form one bond (it has only 1 valence electron).
- Carbon needs 4 bonds to complete its octet.
- Nitrogen needs 3 bonds + 1 lone pair to complete its octet.

So the bonding should be:

- Single bond between H and C
- Triple bond between C and N

That gives:

- H — C (single bond) → uses 2 electrons
- C ≡ N (triple bond) → uses 6 electrons
- Remaining 2 electrons go on nitrogen as a lone pair

✔ So total electrons used: 2 (H–C) + 6 (C≡N) + 2 (lone pair on N) = 10 electrons → matches!

---

## ✘ Problem with the Given Lewis Structure

The image shows:

```
H : C : N :
```

This implies:

- Single bond between H and C (OK)
- Single bond between C and N (WRONG — should be triple)
- Lone pairs shown as dots around atoms, but not correctly placed.

In this drawing:

- Carbon only has 2 bonds (to H and N), so only 4 electrons → violates octet rule.
- Nitrogen has only 1 bond and 3 lone pairs → 8 electrons, which is OK, but carbon is incomplete.
- Also, total bonds shown use only 4 electrons (2 single bonds), leaving 6 electrons as lone pairs — but that doesn’t match the correct bonding.

---

## ✔ Correct Lewis Structure for HCN

```
H — C ≡ N :
↑
(with 2 electrons as lone pair on N)
```

Or drawn with dots:

```
H — C ::: N :
↑
(the three colons represent triple bond; the two dots on N are the lone pair)
```

More clearly:

```
H - C ≡ N
:
```

(Where “:” represents the lone pair on nitrogen)

---

## 📝 Summary of Correction

| Molecule | Total Valence e⁻ | Correct Lewis Structure |
|----------|------------------|--------------------------|
| HCN | 10 | H—C≡N: (with lone pair on N) |

> ⚠️ The structure shown in the table is incorrect. It does not satisfy the octet rule for carbon and misrepresents the bonding.

---

## 💡 Why This Matters

Lewis structures must:

1. Account for all valence electrons.
2. Satisfy the octet rule (for C, N, O, etc.) — except H, which follows duet rule.
3. Minimize formal charges (in this case, HCN has zero formal charges in the correct structure).

---

✔ Final Answer:
The Lewis structure for HCN in the table is incorrect. The correct Lewis structure is H—C≡N:, where carbon is triple-bonded to nitrogen, and nitrogen has one lone pair. This satisfies the octet rule for C and N, and uses all 10 valence electrons.