The VSEPR model is a simple predictive tool that is usually, but not always, correct. Take urea, for instance, a waste product excreted in animal urine:
What hybridization would you expect for the C and N atoms in urea according to the VSEPR model, and what approximate values would you expect for the various bond angles? What are the actual hybridizations and bond angles based on the molecular model shown? 1Red = O, gray = C, blue = N, ivory = H.2

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Identify the central atoms in the urea molecule: carbon (C) and nitrogen (N).

Determine the steric number for each central atom. For carbon, count the number of atoms bonded to it and lone pairs. For nitrogen, do the same.

Use the steric number to predict the hybridization of each central atom. A steric number of 4 indicates sp3 hybridization, 3 indicates sp2, and 2 indicates sp hybridization.

Predict the bond angles based on the hybridization. For sp3, the bond angles are approximately 109.5 degrees; for sp2, they are approximately 120 degrees; and for sp, they are approximately 180 degrees.

Compare the predicted hybridizations and bond angles with the actual molecular model. In the model, observe the geometry around the carbon and nitrogen atoms to determine the actual hybridizations and bond angles.

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주요 개념

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VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the geometry of molecules based on the repulsion between electron pairs in the valence shell of atoms. According to this theory, electron pairs will arrange themselves to minimize repulsion, leading to specific molecular shapes. For urea, this model helps in predicting the arrangement of atoms around the central carbon and nitrogen atoms.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can accommodate the bonding requirements of a molecule. In urea, the carbon atom is typically sp² hybridized, while the nitrogen atom is sp³ hybridized. This hybridization influences the bond angles and the overall geometry of the molecule, which can be analyzed using the VSEPR model.

Bond Angles

Bond angles are the angles formed between adjacent bonds in a molecule, which are influenced by the hybridization of the atoms involved. In urea, the expected bond angles based on the VSEPR model would be approximately 120° for the sp² hybridized carbon and 109.5° for the sp³ hybridized nitrogen. However, actual bond angles may vary slightly due to factors such as lone pair repulsion and the presence of different atoms.