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Magazine Chapter 8: Problem 20
The following molecules or ions all have three oxygen atoms attached to a central atom. Draw a Lewis structure for each one, and then describe the electron-pair geometry and molecular geometry around the central atom. Comment on similarities and differences in the series. (a) \(\mathrm{CO}_{3}^{2-}\) (b) \(\mathrm{NO}_{3}^{-}\) (c) \(\mathrm{SO}_{3}^{2-}\) (d) \(\mathrm{ClO}_{3}^{-}\)
Short Answer
Expert verified
The geometries are: (a) Trigonal planar; (b) Trigonal planar; (c) Trigonal pyramidal; (d) Trigonal pyramidal. Differences arise due to lone pairs.
Step by step solution
01
Draw the Lewis Structure for \( \mathrm{CO}_{3}^{2-} \)
Start by counting the total number of valence electrons. Carbon has 4, each oxygen has 6, and the 2- charge adds 2 more electrons, making a total of 24 valence electrons to place. Position the carbon in the center connected to three oxygen atoms. Distribute the electrons to satisfy oxygen's octet first, then check if carbon achieves octet. One oxygen forms a double bond with carbon, and two form single bonds holding the charge.
02
Describe the Geometry for \( \mathrm{CO}_{3}^{2-} \)
The electron-pair geometry is trigonal planar, as there are three bonding groups around the central atom and no lone pairs. The molecular geometry is the same, trigonal planar. All O-C-O bond angles are approximately 120掳.
03
Draw the Lewis Structure for \( \mathrm{NO}_{3}^{-} \)
Nitrogen has 5 valence electrons, each oxygen has 6, and the ion has a 1- charge adding an extra electron, for a total of 24 valence electrons. Place nitrogen in the center with three oxygens bonded to it. Use one double bond and two single bonds, distributing remaining electrons to satisfy the octets, assigning the negative charge to one of the single-bonded oxygens.
04
Describe the Geometry for \( \mathrm{NO}_{3}^{-} \)
The electron-pair geometry is trigonal planar, as nitrogen is surrounded by three bonding pairs. The molecular geometry is also trigonal planar, similar to carbonate ion; the bond angles are around 120掳.
05
Draw the Lewis Structure for \( \mathrm{SO}_{3}^{2-} \)
Sulfur has 6 valence electrons, each oxygen has 6, and the ion has a 2- charge, resulting in 26 valence electrons. Place sulfur in the center with three oxygens around it. Form three single bonds initially and distribute extra electrons to complete octets. Sometimes, double bonds can be used to minimize formal charges.
06
Describe the Geometry for \( \mathrm{SO}_{3}^{2-} \)
The electron-pair geometry is tetrahedral, considering two lone pairs and three bonding pairs. The molecular geometry, considering only bonded atoms, is trigonal pyramidal. This leads to somewhat smaller bond angles compared to carbonate and nitrate.
07
Draw the Lewis Structure for \( \mathrm{ClO}_{3}^{-} \)
Chlorine has 7 valence electrons, each oxygen has 6, and with the 1- charge, there is a total of 26 electrons to distribute. Chlorine at the center with three oxygens, usually two single bonds and one double bond might be drawn to assign charges correctly and satisfy octets around all, ensuring the overall charge is maintained.
08
Describe the Geometry for \( \mathrm{ClO}_{3}^{-} \)
The electron-pair geometry is tetrahedral because of the lone pair along with three bonding pairs. The molecular geometry is trigonal pyramidal, similar to sulfate, resulting in smaller bond angles compared to trigonal planar geometries.
09
Compare the Geometries
\(\mathrm{CO}_{3}^{2-}\) and \(\mathrm{NO}_{3}^{-}\) both exhibit trigonal planar geometry due to sp\(^2\) hybridization whereas \(\mathrm{SO}_{3}^{2-}\) and \(\mathrm{ClO}_{3}^{-}\) exhibit trigonal pyramidal geometry due to the presence of a lone pair leading to sp\(^3\) hybridization. This highlights the effect of lone pairs vs bonding pairs on geometry.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Electron-Pair Geometry
Electron-pair geometry considers all electron groups, including both bonds and lone pairs, around a central atom.
This helps determine the spatial arrangement that minimizes repulsion between electron groups. For example, small molecules like \(\text{CO}_3^{2-}\) and \(\text{NO}_3^-\) showcase a trigonal planar electron-pair geometry.
This helps determine the spatial arrangement that minimizes repulsion between electron groups. For example, small molecules like \(\text{CO}_3^{2-}\) and \(\text{NO}_3^-\) showcase a trigonal planar electron-pair geometry.
- Trigonal Planar: Occurs when there are three bonding pairs without lone pairs, leading to 120掳 bond angles.
- Tetrahedral: Results from four electron pairs; includes examples with lone pairs like \(\text{SO}_3^{2-}\) and \(\text{ClO}_3^-\).
Molecular Geometry
Molecular geometry describes the actual shape formed by atoms, ignoring any lone pairs. It directly affects properties like polarity and reactivity.
In our examples:
In our examples:
- Trigonal Planar: Seen in \(\text{CO}_3^{2-}\) and \(\text{NO}_3^-\), where three atoms form a flat triangle, thanks to no lone pairs mingling in the central atom's space.
- Trigonal Pyramidal: Observed in \(\text{SO}_3^{2-}\) and \(\text{ClO}_3^-\), where the lone pair distorts the shape, making it a pyramid-like structure with three base atoms.
Valence Electrons
Valence electrons are those in the outermost shell of an atom. They determine how atoms bond and form molecules. Knowing how many valence electrons are involved is essential to draw Lewis structures and predict molecular behavior.
For example:
For example:
- Carbonate \(\text{CO}_3^{2-}\): 4 from carbon and 6 from each oxygen plus 2 extra electrons from charge, totaling 24.
- Nitrate \(\text{NO}_3^-\): 5 from nitrogen, 6 from each oxygen, and one extra due to the negative charge, also totaling 24.
- Sulfate \(\text{SO}_3^{2-}\) and Chlorate \(\text{ClO}_3^-\): More electrons due to sulfur and chlorine's roles, 26 each, when charges are considered.
Hybridization
Hybridization involves the mixing of atomic orbitals to form new hybrid orbitals. This helps explain geometry and bonding properties in molecules.
- sp2 Hybridization: Found in \(\text{CO}_3^{2-}\) and \(\text{NO}_3^-\), forming three identical orbitals on the same plane. This results in a trigonal planar shape.
- sp3 Hybridization: Occurs in \(\text{SO}_3^{2-}\) and \(\text{ClO}_3^-\), where one of the orbitals pairs with a lone pair, forming a tetrahedral arrangement. The lone pair influences the actual molecular shape to be trigonal pyramidal.
