Shapes of molecules, bond angles

VSEPR Theory

1. VSEPR Theory stands for Valence Shell Electron Pair Repulsion Theory.
2. It is used to predict the shape of molecules based on electron pair repulsion.
3. The theory states that electron pairs around a central atom arrange themselves to minimize repulsion.
4. Both bonding pairs and lone pairs of electrons influence molecular geometry.
5. Lone pairs occupy more space than bonding pairs due to greater repulsion.
6. The molecular geometry depends on the number of electron pairs around the central atom.
7. Double bonds and triple bonds are treated as a single region of electron density.
8. The theory provides an approximate shape, which can be refined using experimental data.
9. Examples include linear, trigonal planar, and tetrahedral geometries.
10. VSEPR theory is widely applicable but has limitations for large or complex molecules.

Shapes of Molecules

1. Linear geometry: Found in molecules with 2 regions of electron density, bond angle is 180°. Example: CO₂.
2. Trigonal planar geometry: Found in molecules with 3 regions of electron density, bond angle is 120°. Example: BF₃.
3. Tetrahedral geometry: Found in molecules with 4 regions of electron density, bond angle is 109.5°. Example: CH₄.
4. Trigonal bipyramidal geometry: Found in molecules with 5 regions of electron density, bond angles are 90° and 120°. Example: PCl₅.
5. Octahedral geometry: Found in molecules with 6 regions of electron density, bond angle is 90°. Example: SF₆.
6. Bent geometry: Found in molecules with lone pairs, bond angle is less than the ideal due to lone pair repulsion. Example: H₂O (104.5°).
7. Trigonal pyramidal geometry: Found in molecules with one lone pair, bond angle is slightly less than 109.5°. Example: NH₃.
8. Other geometries include seesaw, T-shaped, and square planar.

Bond Angles

1. Bond angles depend on the number of bonding and non-bonding electron pairs around the central atom.
2. Lone pairs exert more repulsive force than bonding pairs, reducing bond angles.
3. For example, in H₂O, the bond angle is reduced to 104.5° due to lone pairs.
4. In NH₃, the bond angle is 107° due to one lone pair.
5. Bond angles in linear molecules like CO₂ are exactly 180°.
6. In tetrahedral molecules like CH₄, bond angles are 109.5°.
7. Multiple bonds and electronegative atoms can slightly alter bond angles.
8. Deviations in bond angles are also caused by steric factors in bulky groups.

Key Points

1. Linear geometry has a bond angle of 180°.
2. Tetrahedral geometry has a bond angle of 109.5°.
3. Lone pairs reduce bond angles due to greater repulsion.
4. VSEPR theory predicts molecular shapes based on electron pair repulsion.
5. Examples of bent geometry include H₂O and SO₂.
6. In trigonal bipyramidal geometry, axial bonds have 90°, and equatorial bonds have 120°.
7. Octahedral molecules have all bond angles at 90°.
8. Bond angles and shapes are influenced by lone pairs and electronegativity of atoms.
9. Molecules like XeF₂ and XeF₄ have unique geometries due to lone pairs on xenon.
10. Study the geometry of common molecules like CH₄, NH₃, and H₂O for exams.