Predict Molecular Geometry Using VSEPR Theory

❓ Concept Question

How does VSEPR Theory help predict molecular shapes?

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🖼 Concept Image

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✍️ Short Concept

This concept is based on:

👉 VSEPR Theory
👉 Electron pair repulsion
👉 Molecular geometry.

Main idea:

$$\boxed{ \text{Electron pairs repel each other} }$$

and arrange themselves to achieve:

$$\text{minimum repulsion}$$

which gives stable molecular shape.

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🔷 Step 1 — Full Form of VSEPR 💯

VSEPR stands for:

$$\text{Valence Shell Electron Pair Repulsion Theory}$$

According to this theory:

✔ Bond pairs repel each other

✔ Lone pairs repel more strongly

✔ Molecule adjusts shape to minimize repulsion

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🔷 Step 2 — Steps to Predict Shape

To determine molecular geometry:

1. Count valence electrons

Find total valence electrons of central atom.

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2. Count electron pairs

Determine:

✔ Bond pairs (BP)

✔ Lone pairs (LP)

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3. Calculate Steric Number

$$\text{Steric Number}=BP+LP$$

Shape depends on steric number.

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🔷 Step 3 — Common Shapes from Steric Number

Steric Number = 2

$$\boxed{\text{Linear}}$$

Example:

$$BeCl_2$$

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Steric Number = 3

$$\boxed{\text{Trigonal Planar}}$$

Example:

$$BF_3$$

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Steric Number = 4

$$\boxed{\text{Tetrahedral}}$$

Example:

$$CH_4$$

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Steric Number = 5

$$\boxed{\text{Trigonal Bipyramidal}}$$

Example:

$$PCl_5$$

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Steric Number = 6

$$\boxed{\text{Octahedral}}$$

Example:

$$SF_6$$

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🔷 Step 4 — Lone Pair Effect

Lone pairs occupy more space than bond pairs.

Hence:

$$\boxed{ LP-LP > LP-BP > BP-BP }$$

So lone pairs distort molecular shape.

Example:

$$NH_3$$

is pyramidal instead of tetrahedral.

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🔷 Step 5 — AXE Notation Trick 🚨

VSEPR uses:

$$AX_nE_m$$

Where:

• $A$ = central atom
• $X$ = bonded atoms
• $E$ = lone pairs

Example:

$$NH_3=AX_3E$$

This notation helps visualize geometry quickly.

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🔷 Step 6 — JEE Trap Alert 🚨

❌ Lone pair repulsion ignore kar dena

❌ Steric number galat count kar dena

❌ Molecular shape aur electron geometry confuse kar lena

Remember:

$$\boxed{ \text{Lone pairs distort shape} }$$

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✅ Final Takeaway

According to VSEPR theory:

$$\boxed{ \text{Shape depends on electron pair repulsion} }$$

and molecule arranges itself for:

$$\boxed{ \text{minimum repulsion} }$$

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⭐ Golden JEE Insight

For shape prediction:

✔ First calculate steric number

✔ Then identify lone pairs

✔ Finally apply VSEPR chart

This solves most JEE molecular geometry questions quickly.

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📚 Related Topics

• Faraday Law and Nernst Equation Combined Concept
• First Order Growth and Second Order Decay Trick
• Bond Pair and Lone Pair Counting Shortcut