Whatt effect does the presence of lone-pair electro s have om the bond angles in a molecule

Whatt effect does the presence of lone-pair electro s have om the bond angles in a molecule?

The correct answer and explanation is :

The presence of lone pairs of electrons in a molecule affects the bond angles due to the repulsion between the lone pairs and the bonding pairs of electrons. This phenomenon is explained by the Valence Shell Electron Pair Repulsion (VSEPR) Theory, which states that electron pairs around a central atom will arrange themselves in a way that minimizes repulsion.

How Lone Pairs Affect Bond Angles:

Lone pairs of electrons exert a stronger repulsive force compared to bonding pairs. This is because lone pairs are localized closer to the nucleus on the central atom, whereas bonding pairs are shared between atoms and are slightly farther apart. As a result, lone pairs push the bonding pairs closer together, thereby reducing the bond angles between the bonding pairs.

Explanation of Bond Angles:

1. Ideal Bond Angles: In a molecule with no lone pairs on the central atom, the bond angles correspond to the ideal geometry. For example:

• Linear: 180° (e.g., CO₂)
• Trigonal planar: 120° (e.g., BF₃)
• Tetrahedral: 109.5° (e.g., CH₄)

1. Effect of Lone Pairs:

• In a trigonal planar arrangement with one lone pair, the ideal bond angle of 120° is reduced to about 117° because the lone pair repels the bonding pairs more strongly.
• In a tetrahedral arrangement (e.g., CH₄) with one lone pair, the bond angle of 109.5° decreases to around 107°. If there are two lone pairs (e.g., H₂O), the bond angle can decrease further, approaching 104.5°.

1. Why the Angle Decreases: The lone pair-bonding pair repulsion is greater than the bonding pair-bonding pair repulsion. The lone pair’s concentration of electron density pulls the bonding pairs closer, thus compressing the bond angles.

Conclusion:

The presence of lone pairs reduces bond angles because of the greater repulsion they exert compared to bonding pairs. This results in a distortion of the ideal geometry, with bond angles becoming smaller as the number of lone pairs increases.