Bent’s Rule, VSEPR Theory, Coordination Numbers

Bent’s Rule, VSEPR Theory, Coordination Numbers and Geometries

1. Bent’s Rule

• Statement: “Atomic s-character concentrates in orbitals directed toward electropositive substituents, while p-character concentrates in orbitals directed toward more electronegative substituents.”

• Orbitals directed towards electronegative atoms → More p-character.

• Orbitals directed towards electropositive atoms → More s-character.

• Explains bond angles and bond lengths in molecules.

Examples:

CH₃F : C–F bond → more p-character (shorter, stronger); C–H bond → more s-character.
SiHCl₃ : Si–Cl bonds → more p-character; Si–H bond → more s-character.

2. VSEPR Theory

• Basic Idea: Electron pairs around the central atom arrange themselves in space to minimize repulsion.

• Lone pairs repel more strongly than bonding pairs → distortions in geometry.

Typical Geometries:

Steric No.	Arrangement	Geometry	Example
2	Linear	Linear	BeCl₂
3	Trigonal planar	Trigonal planar	BF₃
4	Tetrahedral	Tetrahedral	CH₄
4	Trigonal pyramid	NH₃
4	Bent (angular)	H₂O
5	Trigonal bipyramidal	PCl₅
5	See-saw	SF₄
6	Octahedral	SF₆
6	Square pyramidal	BrF₅
6	Square planar	XeF₄

3. Coordination Numbers and Geometries

• The coordination number (CN) of a central atom is the number of atoms/ligands directly bonded to it.

CN = 2 : Linear geometry → [Ag(NH₃)₂]⁺
CN = 3 : Trigonal planar → [HgI₃]^-
CN = 4 : Tetrahedral → [ZnCl₄]^2-; Square planar → [Ni(CN)₄]^2-
CN = 5 : Trigonal bipyramidal → [Fe(CO)₅]; Square pyramidal → [CuCl₅]^3-
CN = 6 : Octahedral → [Co(NH₃)₆]³⁺
CN = 7 : Pentagonal bipyramidal → [ZrF₇]^3-
CN = 8 : Square antiprismatic/cubic → [Mo(CN)₈]^4-
CN = 9 : Tricapped trigonal prismatic → [ReH₉]^2-

Summary

• Bent’s Rule explains hybridization distribution.

• VSEPR Theory predicts molecular shape based on electron repulsion.

• Coordination number defines how many ligands are directly attached, leading to specific geometries.

Atomic Structure -Bent's rule, VSEPR theory (typical example) co-ordination numbers and geometries.

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