3.5.1 Shapes of Molecules Containing Two Electron Pairs

This topic explains the shape of molecules where the central atom is surrounded by two electron pairs and no lone pairs.

Definition

Two Electron Pairs (AX₂): A molecule in which the central atom is surrounded by two bonding pairs of electrons and no lone pairs.

Main Idea

When a central atom is surrounded by two electron pairs and no lone pairs, the electron pairs arrange themselves as far apart as possible to minimize repulsion. This gives the molecule a linear shape with a bond angle of 180°.

Core Concept

According to VSEPR theory, electron pairs repel each other. For two electron pairs, the arrangement that minimizes repulsion is 180° apart, resulting in a linear geometry. The general formula for such molecules is AX₂, where A is the central atom and X represents the bonded atoms.

Detailed Explanation

Shape: Linear

• The central atom is surrounded by two bonding pairs.
• There are no lone pairs on the central atom.
• To minimize repulsion, the two electron pairs are placed 180° apart.
• The shape of the molecule is linear.
• The bond angle is 180°.

Example: Beryllium Fluoride (BeF₂)

• Beryllium (Be) has atomic number 4.
• Ground state configuration: 1s² 2s²
• In the excited state, one 2s electron is promoted to a 2p orbital: 1s² 2s¹ 2p¹

• The central atom, beryllium, contributes two valence electrons.
• Each fluorine atom contributes one valence electron.
• Beryllium forms two sigma bonds with two fluorine atoms.
• The Lewis electron structure is: :F - Be - F:

• There are two electron pairs around the central beryllium atom.
• The arrangement that minimizes repulsions places the groups 180° apart.
• The shape of BeF₂ is linear.
• The bond angle is 180°.

General Formula: AX₂

Other Examples

Note: CO₂ has double bonds, but the electron pair geometry is still linear because there are only two electron regions around the central carbon atom.

Intermediate Level

AX₂ Molecules

• Central atom: A
• Two bonded atoms: X
• No lone pairs on A
• Shape: Linear
• Bond angle: 180°

Example: BeF₂

• Central atom: Be
• Valence electrons of Be: 2
• Each F contributes 1 electron
• Total electron pairs around Be: 2
• Both are bonding pairs (no lone pairs)
• Arrangement: 180° apart
• Shape: Linear

Table of AX₂ Molecules

Important Points

• 1. AX₂ molecules have two bonding pairs and no lone pairs around the central atom.
• 2. The shape is linear.
• 3. The bond angle is 180°.
• 4. Example: BeF₂, BeCl₂, CO₂, HgCl₂.
• 5. Beryllium undergoes sp hybridization in BeF₂.
• 6. The two sp hybrid orbitals are arranged linearly at 180°.

Remember

• - AX₂: Two bonding pairs, no lone pairs
• - Shape: Linear
• - Bond angle: 180°
• - Example: BeF₂
• - Hybridization: sp
• - Orbital overlap: sp-p sigma bond

Common Mistakes

• 1. Thinking AX₂ molecules have a bent shape: They are linear.
• 2. Forgetting the bond angle: It is 180°.
• 3. Confusing electron pair geometry with molecular geometry: For AX₂, both are linear.
• 4. Ignoring hybridization: Beryllium undergoes sp hybridization in BeF₂.

Quick Revision

• AX₂ molecules have two bonding pairs and no lone pairs around the central atom.
• The shape is linear.
• The bond angle is 180°.
• Example: BeF₂, BeCl₂, CO₂, HgCl₂.
• Beryllium undergoes sp hybridization in BeF₂.
• The two sp hybrid orbitals are arranged linearly at 180°.

60-Second Revision

• AX₂: Two bonding pairs, no lone pairs
• Shape: Linear
• Bond angle: 180°
• Example: BeF₂
• Hybridization: sp
• Orbital overlap: sp-p sigma bond

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