Question

Discuss the shape of the following molecules using the VSEPR model: 
BeCl₂, BCl₃, SiCl₄, AsF₅, H₂S, PH₃

Answer 1

VSEPR Model:

According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, the shape of a molecule depends on the number of electron pairs (bond pairs and lone pairs) around the central atom. These electron pairs arrange themselves to minimise repulsion.

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(i) BeCl₂

• Central atom Be has 2 bond pairs and 0 lone pairs.
• Electron pair geometry is linear.
• Bond angle = 180°.

Shape: Linear

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(ii) BCl₃

• Central atom B has 3 bond pairs and 0 lone pairs.
• Electron pairs arrange in a trigonal planar manner.
• Bond angle = 120°.

Shape: Trigonal planar

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(iii) SiCl₄

• Central atom Si has 4 bond pairs and 0 lone pairs.
• Electron pairs arrange tetrahedrally.
• Bond angle = 109.5°.

Shape: Tetrahedral

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(iv) AsF₅

• Central atom As has 5 bond pairs and 0 lone pairs.
• Electron pairs arrange in a trigonal bipyramidal geometry.

Shape: Trigonal bipyramidal

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(v) H₂S

• Central atom S has 2 bond pairs and 2 lone pairs.
• Lone pair–lone pair repulsion is strong.
• Bond angle is less than tetrahedral.

Shape: Bent or V-shaped

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(vi) PH₃

• Central atom P has 3 bond pairs and 1 lone pair.
• Lone pair–bond pair repulsion slightly reduces bond angle.

Shape: Trigonal pyramidal

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Conclusion:

The VSEPR model successfully explains the shapes of molecules based on repulsions between electron pairs around the central atom.