Short Answer

Bond angle is the angle formed between two bonds that originate from the same central atom in a molecule. It tells us how far apart the bonded atoms are positioned around the central atom. Bond angle helps determine the shape and geometry of the molecule.

Different molecules have different bond angles depending on the number of electron pairs, lone pairs, and type of hybridization. Repulsion between electron pairs plays a major role—the stronger the repulsion, the larger or smaller the bond angle becomes. Bond angle is important for predicting molecular structure and chemical behaviour.

Detailed Explanation :

Bond Angle

Bond angle is a key concept in molecular geometry. It describes the angle formed between two bonds that originate from the same central atom. Bond angle gives a clear idea about the shape of a molecule and how atoms are arranged in space. Each molecule has a characteristic set of bond angles based on the number of electron pairs around the central atom and the repulsions between them. Understanding bond angles helps us predict molecular shape, polarity, reactivity, and physical properties.

Bond angle is measured in degrees (°). It is determined mainly by electron pair repulsion, the type of hybridization, the presence of lone pairs, and the size of the surrounding atoms. In general, electron pairs try to stay as far apart as possible, resulting in specific and predictable bond angles.

1. Role of Electron Pair Repulsion

According to VSEPR theory (Valence Shell Electron Pair Repulsion), electron pairs around a central atom repel each other and arrange themselves to minimize repulsion. This arrangement determines the bond angle.

Different electronic arrangements give characteristic bond angles:

• Linear (AX₂) → 180°
• Trigonal planar (AX₃) → 120°
• Tetrahedral (AX₄) → 109.5°
• Trigonal bipyramidal (AX₅) → 90° and 120°
• Octahedral (AX₆) → 90°

These ideal angles can be modified by lone pairs or differences in atom sizes.

2. Effect of Lone Pairs on Bond Angle

Lone pairs occupy more space than bonding pairs because they are held closer to the nucleus and exert stronger repulsion.

Repulsion strength order:
Lone pair–lone pair > Lone pair–bond pair > Bond pair–bond pair

Therefore:

• Lone pairs decrease bond angle by pushing bonding pairs closer together.

Examples:

• Water (H₂O): The ideal tetrahedral angle is 109.5°, but due to 2 lone pairs, the bond angle reduces to 104.5°.
• Ammonia (NH₃): One lone pair reduces the angle from 109.5° to 107°.

Thus, more lone pairs → smaller bond angle.

3. Effect of Bond Order

Bond order affects bond angle because double and triple bonds create stronger repulsion than single bonds.

Repulsion strength:
Triple bond > Double bond > Single bond

Example:

• In formaldehyde (H₂CO), the double bond causes larger repulsion, so the H–C–H angle becomes slightly greater than 120°.

Higher bond order increases electron density, which slightly increases bond angle.

4. Effect of Hybridization on Bond Angle

Hybridization influences bond angle because hybrid orbitals orient themselves to achieve maximum separation.

Bond angles in different hybridizations:

• sp hybridization → 180°
• sp² hybridization → 120°
• sp³ hybridization → 109.5°

Reason:
Higher s-character leads to shorter and stronger bonds that open up angles.

Thus:
sp (50% s) gives the largest angle,
sp² (33% s) gives medium angle,
sp³ (25% s) gives smallest angle.

5. Effect of Atom Size

Larger atoms around the central atom require more space. Thus:

• Bigger surrounding atoms increase bond angles.
• Smaller atoms may reduce bond angles.

Example:
In PCl₃ vs. PF₃:
Fluorine is smaller, so PF₃ has a smaller bond angle because of stronger lone-pair repulsion on phosphorus.

6. Effect of Electronegativity

Electronegative atoms pull electrons toward themselves more strongly.

• High electronegativity → reduces repulsion near the central atom → decreases bond angle
• Low electronegativity → increases bond angle

Example:
NH₃ (bond angle 107°) vs. NF₃ (bond angle 102°):
F pulls electrons away more strongly, reducing repulsion at the nitrogen center.

7. Effect of Multiple Bonds and Pi Bonds

Multiple bonds contain pi bonds, which have higher electron density.

• Pi electrons repel bonding electrons → may increase bond angle
• Example: C=C double bonds cause wider angles in alkenes

Pi bonds add rigidity that also restricts rotation, affecting molecular geometry.

8. Molecular Environment

Bond angles can slightly vary depending on:

• Phase (gas, solid, liquid)
• Intermolecular forces
• Pressure and temperature

However, these changes are usually small compared to electronic effects.

Conclusion

Bond angle is the angle formed between two bonds originating from the same central atom, and it is a key factor in determining molecular shape. It is influenced by electron pair repulsion, hybridization, size of surrounding atoms, presence of lone pairs, electronegativity, and multiple bonds. Understanding bond angles helps predict molecular geometry and chemical behaviour, making it essential in studying the structure of molecules.