Bond Pair and Lone Pair Counting Shortcut

 

❓ Concept Question

How do we quickly find the ratio of:

$$\text{Bond Pairs : Lone Pairs}$$

on the central atom in molecules and ions?

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🖼 Concept Image

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✍️ Short Concept

Always count:

✅ Sigma bonds only
✅ Lone pairs only on central atom

⚠️ Double bond counts as ONE sigma bond.

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🔷 Step 1 — Golden Rule 💯

For central atom:

$$\text{Bond Pair : Lone Pair}$$

Count:

• Sigma bonds
• Lone pairs

Only on CENTRAL atom.

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🔷 Step 2 — $ICl_2^-$

Valence electrons on I:

$$7 + 1 = 8$$

Structure:

• 2 bond pairs
• 3 lone pairs

So:

$$\boxed{2:3}$$

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🔷 Step 3 — $H_2O$

Oxygen valence electrons:

$$6$$

Structure:

• 2 bond pairs
• 2 lone pairs

So:

$$\boxed{{\displaystyle 2:2}}$$

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🔷 Step 4 — $SO_2$

Sulfur valence electrons:

$$6$$

Important:

Double bonds count as ONE sigma bond each.

So central sulfur has:

• 2 bond pairs
• 1 lone pair

Ratio:

$$\boxed{2:1}$$

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🔷 Step 5 — $XeF_4$

Xenon valence electrons:

$$8$$

Structure:

• 4 bond pairs
• 2 lone pairs

Ratio:

$$\boxed{4:2}$$

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✅ Final Takeaway

Always remember:

$$\boxed{\text{Double bond = ONE sigma bond}}$$

Then:

👉 Count sigma bonds
👉 Count lone pairs
👉 Write ratio.

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⭐ Golden JEE Insight

Most mistakes happen when students:

❌ Count π bonds separately
❌ Count lone pairs on surrounding atoms

Correct rule:

👉 Only central atom matters.