Use VSEPR theory to answer the following questions: (a) Which molecule, BF3 or PF3, has the smaller F¬X¬F angles?

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Identify the central atom in each molecule: In BF3, the central atom is Boron (B), and in PF3, the central atom is Phosphorus (P).

Determine the electron domain geometry around the central atom using VSEPR theory. For BF3, Boron has three bonding pairs and no lone pairs, leading to a trigonal planar shape. For PF3, Phosphorus has three bonding pairs and one lone pair, resulting in a trigonal pyramidal shape.

Understand the effect of lone pairs on bond angles: Lone pairs repel more strongly than bonding pairs, which tends to compress the bond angles.

Compare the bond angles: In BF3, the absence of lone pairs means the F-B-F bond angles are 120 degrees, typical of a trigonal planar geometry. In PF3, the presence of a lone pair on Phosphorus compresses the F-P-F bond angles to less than 120 degrees.

Conclude which molecule has smaller F-X-F angles: Since PF3 has a lone pair that compresses the bond angles, PF3 has smaller F-X-F angles compared to BF3.

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주요 개념

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VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the geometry of individual molecules based on the repulsion between electron pairs in the valence shell of the central atom. According to VSEPR, electron pairs, whether bonding or lone pairs, will arrange themselves to minimize repulsion, leading to specific molecular shapes and bond angles.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. The shape of a molecule influences its physical and chemical properties, including bond angles. In the case of BF3 and PF3, the central atom's size and the number of lone pairs affect the overall geometry and the resulting F-X-F bond angles.

Bond Angles

Bond angles are the angles formed between two adjacent bonds at a central atom. They are influenced by the molecular geometry and the presence of lone pairs. In BF3, the bond angles are expected to be larger due to the absence of lone pairs on boron, while PF3 has a lone pair on phosphorus, which can compress the bond angles, resulting in smaller F-P-F angles compared to F-B-F angles.