Chapter 11: Problem 87
Predict the geometric structure of the carbonate ion, \(\mathrm{CO}_{3}^{2-} .\) What are the bond angles in this molecule?
Short Answer
Expert verified
The geometric structure of the carbonate ion, \(\mathrm{CO}_{3}^{2-}\), is trigonal planar, and the bond angles are 120°.
Step by step solution
01
Identify the central atom
The central atom in the carbonate ion is carbon (C), with oxygen (O) atoms surrounding it.
02
Determine the electron pairs around the central atom
First, determine the number of valence electrons for C and O atoms. Since C is in Group 4A, it has 4 valence electrons. Each O atom is in Group 6A and has 6 valence electrons. However, because of the 2- charge on the ion, we also need to add 2 more electrons. The total valence electrons are:
\(4 + 3\times 6 + 2 = 24\)
Carbon is the central atom, and it is bonded to 3 oxygen atoms. Each bond consumes 2 electrons, leaving 24 - 6 = 18 non-bonding electrons, which will be distributed among the oxygen atoms.
03
Distribute the non-bonding electrons
Divide the non-bonding electrons evenly among the 3 oxygen atoms. By following the octet rule, each oxygen atom will have 6 non-bonding electrons forming 3 lone pairs:
Oxygen 1: 6 non-bonding electrons
Oxygen 2: 6 non-bonding electrons
Oxygen 3: 6 non-bonding electrons
04
Determine the electron domain geometry
Carbon has 3 bonding electron domains and no non-bonding electron domains. According to the VSEPR theory, 3 electron domains result in a trigonal planar electron domain geometry.
05
Determine the molecular geometry
Since there are no lone pairs on the central carbon atom in the \(\mathrm{CO}_{3}^{2-}\) ion, the molecular geometry will be the same as the electron domain geometry, which is trigonal planar.
06
Determine the bond angles
In a trigonal planar geometry, the bond angles are equal to 120°.
So, the geometric structure of the carbonate ion, \(\mathrm{CO}_{3}^{2-}\), is trigonal planar, and the bond angles are 120°.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
VSEPR Theory
Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used in chemistry to predict the geometry of individual molecules based on the number of electron pairs surrounding their central atoms. The main idea behind VSEPR theory is that electron pairs located in the valence shell of an atom will repel each other. Because these pairs want to be as far apart as possible, they end up pushing away from each other, which determines the shape of the molecule.
When applying VSEPR theory, each pair of bonded electrons forms a 'domain', as do lone pairs of electrons. Domains can be bonding or non-bonding. Bonding domains consist of shared electrons (in bonds), while non-bonding domains are unshared lone pairs. The repulsion between these domains dictates the optimal arrangement of atoms around the central atom, which minimizes the repulsion and, hence, creates the molecule's shape. For instance, when we determine the structure of the carbonate ion, \(\mathrm{CO}_{3}^{2-}\), we first count the electron domains to apply the VSEPR theory.
The theory prioritizes repulsions in the following order: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair. This hierarchy is essential as it affects the resultant geometry of the molecule. In summary, VSEPR theory provides a useful, straightforward approach to understanding and predicting the shapes of molecules.
When applying VSEPR theory, each pair of bonded electrons forms a 'domain', as do lone pairs of electrons. Domains can be bonding or non-bonding. Bonding domains consist of shared electrons (in bonds), while non-bonding domains are unshared lone pairs. The repulsion between these domains dictates the optimal arrangement of atoms around the central atom, which minimizes the repulsion and, hence, creates the molecule's shape. For instance, when we determine the structure of the carbonate ion, \(\mathrm{CO}_{3}^{2-}\), we first count the electron domains to apply the VSEPR theory.
The theory prioritizes repulsions in the following order: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair. This hierarchy is essential as it affects the resultant geometry of the molecule. In summary, VSEPR theory provides a useful, straightforward approach to understanding and predicting the shapes of molecules.
Trigonal Planar Geometry
Trigonal planar geometry is a term used to describe the shape of a molecule when a central atom is surrounded by three atoms at angles of 120° to each other, which lie in the same plane. This geometry is a consequence of a central atom having exactly three bonding electron domains, with no lone pairs to cause additional repulsion or distort the shape.
When a molecule exhibits trigonal planar geometry, it is flat, with each of its outer atoms equidistant from the neighboring atoms. The perfect example to illustrate this concept is the carbonate ion, which we analyze in our exercise. Here, the carbon (C) is the central atom bonded to three oxygen (O) atoms, and all four atoms reside in the same plane. The three electron domains formed by the C-O bonds spread out in the plane, providing the molecule a flat, triangular shape.
How do these geometries influence molecule properties? Firstly, the planar shape can affect the molecule's polarity. If the surrounding atoms are identical and there is symmetry, the molecule might be nonpolar. Secondly, this geometry can influence the molecule's reactivity by making certain atoms more accessible to reactants. Understanding the implications of trigonal planar geometry is important in the study of molecular structures and reactivity.
When a molecule exhibits trigonal planar geometry, it is flat, with each of its outer atoms equidistant from the neighboring atoms. The perfect example to illustrate this concept is the carbonate ion, which we analyze in our exercise. Here, the carbon (C) is the central atom bonded to three oxygen (O) atoms, and all four atoms reside in the same plane. The three electron domains formed by the C-O bonds spread out in the plane, providing the molecule a flat, triangular shape.
How do these geometries influence molecule properties? Firstly, the planar shape can affect the molecule's polarity. If the surrounding atoms are identical and there is symmetry, the molecule might be nonpolar. Secondly, this geometry can influence the molecule's reactivity by making certain atoms more accessible to reactants. Understanding the implications of trigonal planar geometry is important in the study of molecular structures and reactivity.
Electron Domain Geometry
Electron domain geometry refers to the three-dimensional arrangement of electron domains (bonding and non-bonding pairs of electrons) around a central atom in a molecule. It's an essential concept in predicting the layout of a molecule using VSEPR theory. The shape is determined by the total number of electron domains, regardless of whether they are bond pairs or lone pairs.
In case of the carbonate ion (\(\mathrm{CO}_{3}^{2-}\)), we observe that the carbon has three regions of electron density that arrange themselves in a plane to minimize repulsion, resulting in a trigonal planar electron domain geometry. This is slightly contrasted with molecular geometry, which may differ and is influenced by the actual number of bonding domains versus lone pairs on the central atom.
The term 'electron domain geometry' is sometimes used interchangeably with 'molecular geometry,' but they are not always synonymous. Electron domain geometry provides the general framework of the molecule, while molecular geometry can often be more specific, particularly when considering the influence of lone pair repulsion. For instance, if lone pairs are present, they can alter the geometry because they occupy more space than bonding pairs, leading to adjustments in the angles between the domains. In the context of our example, since the central carbon atom has no lone pairs, the electron domain geometry and molecular geometry coincide to be trigonal planar.
In case of the carbonate ion (\(\mathrm{CO}_{3}^{2-}\)), we observe that the carbon has three regions of electron density that arrange themselves in a plane to minimize repulsion, resulting in a trigonal planar electron domain geometry. This is slightly contrasted with molecular geometry, which may differ and is influenced by the actual number of bonding domains versus lone pairs on the central atom.
The term 'electron domain geometry' is sometimes used interchangeably with 'molecular geometry,' but they are not always synonymous. Electron domain geometry provides the general framework of the molecule, while molecular geometry can often be more specific, particularly when considering the influence of lone pair repulsion. For instance, if lone pairs are present, they can alter the geometry because they occupy more space than bonding pairs, leading to adjustments in the angles between the domains. In the context of our example, since the central carbon atom has no lone pairs, the electron domain geometry and molecular geometry coincide to be trigonal planar.
