Question

Draw a Lewis structure for the urea molecule, \(\mathrm{CO}\left(\mathrm{NH}_{2}\right)_{2},\) and predict its geometric shape with the VSEPR theory. Then revise your assessment of this molecule, given the fact that all the atoms lie in the same plane, and all the bond angles are \(120^{\circ} .\) Propose a hybridization and bonding scheme consistent with these experimental observations.

Short answer

The urea molecule \(\mathrm{CO}\left(\mathrm{NH}_{2}\right)_{2}\) has a Lewis structure where carbon is double bonded to oxygen and single-bonded to the nitrogen atoms (which are each single-bonded to two hydrogen atoms). The initial prediction of a tetrahedral molecular shape (based on VSEPR theory) is revised to a trigonal planar shape to match the given experimental data. The carbon atom achieves this shape through sp2 hybridization, suggesting a bonding scheme where each sp2 hybrid orbital forms a sigma bond with the oxygen or nitrogen atoms, and the remaining 2p orbital forms a pi bond with the oxygen atom.

Step-by-step solution

Draw the Lewis Structure

A Lewis structure shows the placement of atoms and respective lone pairs and bonding pairs of electrons. The important part is finding how many valence electrons are available and placing these electrons around the atoms. The urea molecule, \(\mathrm{CO}\left(\mathrm{NH}_{2}\right)_{2}\), is composed of one carbon atom, one oxygen atom, and two nitrogen atoms, each bonded to two hydrogen atoms. The number of valence electrons totals to 24. When you place these electrons around the atoms, keep the rule of octet in mind. This would result in the formation of double bond between carbon and oxygen atoms, and single bonds between nitrogen and hydrogen atoms, as well as carbon and nitrogen atoms.

Predict the Shape Using VSEPR Theory

The Valence-Shell Electron-Pair Repulsion (VSEPR) theory allows us to predict the shape of molecules based on their Lewis structure. The electron pairs around the central atom lead to a tetrahedral shape according to VSEPR theory because carbon is the central atom and has four areas of electron density (the oxygen atom and three nitrogen atoms).

Revise the Shape to Fit Given Data

However, this prediction doesn't match up with the given experimental observations. It's been given that all atoms lie in the same plane and the bond angles are 120 degrees. This suggests that the molecule has a trigonal planar shape.

Propose a Hybridization and Bonding Scheme

Hybridization refers to the combining of atomic orbitals to form the same number of equivalent hybrid orbitals. To achieve a trigonal planar structure with 120-degree bond angles, the central carbon atom would need to be sp2 hybridized. Under this scheme, there would be one 2s orbital and two 2p orbitals combining to form three sp2 hybrid orbitals, with each one forming a sigma bond with the oxygen atom or a nitrogen atom. The remaining unhybridized 2p orbital on the carbon atom will form a pi bond with the oxygen atom, completing the double bond.

Key concepts

VSEPR Theory

In chemistry, understanding the shape of molecules is crucial as it affects their reactivity and interactions. The Valence-Shell Electron-Pair Repulsion (VSEPR) theory provides a straightforward way to predict the geometry of a molecule based on its Lewis structure. VSEPR theory posits that electron pairs, whether in bonds or as lone pairs, repel each other and arrange themselves as far apart as possible around the central atom. This arrangement minimizes the repulsion between them.
For instance, in the urea molecule, initially, one might predict a tetrahedral arrangement around the central carbon. This prediction comes from the observation of four areas of electron density (one double-bonded oxygen and three points connected to the nitrogen atoms with single bonds). However, experimental observations prompt revisiting this initial prediction. Despite the typical tetrahedral prediction, given the information that all atoms in the urea molecule lie in the same plane and the bond angles measure 120°, it aligns better with a trigonal planar shape. This discrepancy highlights how real-life conditions can impact molecular geometry predictions.
The VSEPR model provides a starting point, but it's essential to correlate these predictions with actual experimental data for accurate assessments.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals. These hybrids describe the bonding in molecules more accurately than the original atomic orbitals. The process of hybridization helps explain molecular shapes predicted by VSEPR theory.
For urea, while considering the experimental data that shows the molecule is planar with 120-degree bond angles, you find that the central carbon atom must be sp2 hybridized. This type of hybridization involves mixing one 2s orbital and two 2p orbitals from carbon to form three sp2 hybrid orbitals. These orbitals arrange themselves at 120° angles to each other, aligning with the planar shape and bond angles observed in urea.
The sp2 hybridized orbitals of carbon form sigma bonds with the oxygen and nitrogen atoms. Meanwhile, the unhybridized 2p orbital on carbon creates a pi bond with oxygen, resulting in the double bond seen in the Lewis structure. Hybridization not only helps explain the molecule's geometry but also its electronic structure, providing a deeper understanding of bonding properties.

Trigonal Planar Geometry

The trigonal planar geometry is characterized by three atoms attached to a central atom, all lying within the same plane, forming 120-degree bond angles. This shape is typical of molecules where the central atom is sp2 hybridized, such as in urea.
Urea's trigonal planar shape comes into play with carbon being at the center, bonded to one oxygen atom and two nitrogen atoms. This geometry is evident from the fact that the molecule lies flat, and its bond angles are all approximately 120°, as opposed to the 109.5° angles typical of a tetrahedral structure.
This geometry can have significant implications for the molecule's properties, affecting how it interacts with other molecules and its reactivity. The flatness and arrangement are crucial in chemical reactions, where the planarity might enable better overlapping of orbitals between reacting species, allowing for something like resonance in more complex systems.
Hence, appreciating the specific geometry of a molecule like urea allows us to predict its behavior in chemical pathways and interactions efficiently.