Molecular Geometry: Electron Groups and Molecular Shapes

Concept: Molecular Geometry (Simplified)

Introduction to Molecular Geometry

The shape of a molecule is determined by the arrangement of electron groups (bonding pairs and lone pairs) around the central atom. The repulsion between these groups leads to specific molecular geometries. This concept is essential for predicting the physical and chemical properties of molecules.

• Electron groups include both bonding pairs (single, double, or triple bonds) and lone pairs of electrons.

• The arrangement minimizes repulsion, leading to characteristic shapes.

Two Electron Groups

Linear Geometry

Central atoms with two electron groups (either bonding or lone pairs) adopt a linear geometry to minimize repulsion.

Electron Groups	Bonding Groups	Lone Pairs	Lewis Structure Example	Molecular Geometry	Bond Angle
2	2	0	CO2	Linear	180°

• Example: Carbon dioxide (CO2) is linear because there are two double bonds and no lone pairs on the central atom.

Three Electron Groups

Trigonal Planar and Bent Geometries

Central atoms with three electron groups can have zero or one lone pair, resulting in two possible molecular geometries.

Electron Groups	Bonding Groups	Lone Pairs	Lewis Structure Example	Molecular Geometry	Bond Angle
3	3	0	BF3	Trigonal planar	120°
3	2	1	SO2	Bent	<120°

• Example: Boron trifluoride (BF3) is trigonal planar; sulfur dioxide (SO2) is bent due to one lone pair.

Four Electron Groups

Tetrahedral, Trigonal Pyramidal, and Bent Geometries

Central atoms with four electron groups can have zero, one, or two lone pairs, resulting in three possible molecular geometries.

Electron Groups	Bonding Groups	Lone Pairs	Lewis Structure Example	Molecular Geometry	Bond Angle
4	4	0	CH4	Tetrahedral	109.5°
4	3	1	NH3	Trigonal pyramidal	<109.5°
4	2	2	H2O	Bent	<109.5°

• Example: Methane (CH4) is tetrahedral; ammonia (NH3) is trigonal pyramidal; water (H2O) is bent.

Examples and Practice

Worked Example: BCl3

• Step 1: Count electron groups around the central atom (Boron in BCl3).

• Step 2: Boron has three bonding pairs and no lone pairs.

• Step 3: Geometry is trigonal planar.

Worked Example: NH4+

• Step 1: Count electron groups around the central atom (Nitrogen in NH4+).

• Step 2: Nitrogen has four bonding pairs and no lone pairs.

• Step 3: Geometry is tetrahedral.

Practice Problem

• Determine the molecular geometry for the following molecule: FSSF.

• Hint: Follow the steps above to count electron groups and assign geometry.

Summary Table: Electron Groups and Molecular Geometry

Electron Groups	Lone Pairs	Molecular Geometry	Bond Angle
2	0	Linear	180°
3	0	Trigonal planar	120°
3	1	Bent	<120°
4	0	Tetrahedral	109.5°
4	1	Trigonal pyramidal	<109.5°
4	2	Bent	<109.5°

Key Equations

• Bond angle for linear geometry: $180^\circ$

• Bond angle for trigonal planar geometry: $120^\circ$

• Bond angle for tetrahedral geometry: $109.5^\circ$

Additional info: These geometries are predicted by the Valence Shell Electron Pair Repulsion (VSEPR) theory, which is fundamental for understanding molecular shapes in chemistry.