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Magazine Chapter 10: Problem 35
Write a Lewis structure for each of the following species. Indicate all of the bond angles as predicted by the VSEPR model. Deduce the skeleton structure from the way each formula is written. (c) \(\mathrm{SCN}^{-}\) (a) \(\mathrm{SO}_{2}\) (b) \(\mathrm{ClO}_{3}^{-}\) (
Short Answer
Expert verified
SCN^- is linear with 180掳 bond angle, SO鈧� is bent with ~120掳, ClO鈧冣伝 is trigonal pyramidal with ~107掳.
Step by step solution
01
Evaluate Valence Electrons for SCN^-
The molecule SCN^- consists of sulfur (S), carbon (C), and nitrogen (N) with a negative charge. Determine the total number of valence electrons:
- Sulfur has 6 valence electrons.
- Carbon has 4 valence electrons.
- Nitrogen has 5 valence electrons.
- The negative charge adds 1 extra electron.
Adding those gives us a total of 6 + 4 + 5 + 1 = 16 valence electrons.
02
Draw Lewis Structure for SCN^-
Arrange the atoms in a linear arrangement with carbon in the center: S-C-N.
1. Place a single bond between S-C and C-N, using 2 electrons for each bond.
2. S=C=N is a typical structure for SCN^-.
3. Distribute remaining electrons to complete octets, giving N a triple bond with C and a lone pair, and C a double bond with S.
This structure uses all 16 electrons.
03
Determine Bond Angles for SCN^-
SCN^- is a linear molecule as indicated by its skeleton structure. Therefore, the bond angle for S-C-N is 180掳.
04
Evaluate Valence Electrons for SO_2
For SO鈧�:
- Sulfur contributes 6 valence electrons.
- Each oxygen contributes 6 valence electrons.
This results in 6 + 2(6) = 18 valence electrons total.
05
Draw Lewis Structure for SO_2
1. Place sulfur in the center with oxygen atoms on either side.
2. Connect sulfur to each oxygen with a single bond, using 4 electrons.
3. Distribute remaining electrons, prioritizing octets around oxygen.
4. Form additional bonds to ensure octets are fulfilled, resulting in double bonds on each side: O=S=O.
06
Determine Bond Angles for SO_2
SO鈧� has a bent shape because the double bonds create two regions of electron density around sulfur. According to the VSEPR model, this gives a bond angle about 120掳.
07
Evaluate Valence Electrons for ClO_3^-
For ClO鈧冣伝:
- Chlorine contributes 7 valence electrons.
- Each oxygen contributes 6 valence electrons.
- The negative charge adds 1 electron.
Total electrons: 7 + 3(6) + 1 = 26 electrons.
08
Draw Lewis Structure for ClO_3^-
1. Central chlorine atom with three oxygen atoms surrounding it.
2. Form single bonds from Cl to each O, using 6 electrons.
3. Distribute remaining electrons to fulfill octets.
4. Add double bond to one oxygen to reduce formal charges, resulting in Cl with one double-bonded oxygen and two single-bonded oxygens, each with complete octets.
09
Determine Bond Angles for ClO_3^-
ClO鈧冣伝 has a trigonal pyramidal shape due to the electron repulsion from the lone pair on chlorine, leading to bond angles of approximately 107掳.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Valence Electrons
Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding. They determine how elements interact and bond with other atoms. Here鈥檚 how to evaluate them for typical molecules:
- Identify the element and its group number in the periodic table. This is typically the same as the number of valence electrons an element has. For example, oxygen is in group 16, so it has 6 valence electrons.
- Account for charge. A negative charge indicates an additional electron, while a positive charge indicates one less electron.
- For polyatomic ions like \(\text{SCN}^-\), sum the valence electrons of all constituent atoms and adjust for any charge to find the total valence electron count. In this example, adding the electrons results in 16 valence electrons due to the combination of sulfur, carbon, nitrogen, and the additional charge from the negative ion.
VSEPR Model
The VSEPR model (Valence Shell Electron Pair Repulsion) is a tool used to predict the geometry of molecules based on electron repulsion. This model helps us understand the three-dimensional shape of molecules by assuming that electron pairs will arrange themselves to minimize repulsion.
- Identify the central atom in the molecule, generally the least electronegative atom.
- Count the regions of electron density around the central atom, which include bonded atoms and lone pairs. Each region will try to stay as far apart as possible.
- Use VSEPR theory to determine the molecular geometry:
- For example, the \(\text{SO}_2\) molecule is bent due to two bonded electron pairs and one lone pair creating electron repulsion.
- \(\text{ClO}_3^-\) features a trigonal pyramidal shape due to one lone pair on the central chlorine atom, affecting the angle between bonded oxygen atoms.
Bond Angles
Bond angles are the angles formed between adjacent bonds of a molecule鈥檚 central atom. Understanding them helps describe the molecule's shape and predict its chemical properties. Bond angles depend on the number of electron pairs around the central atom and their repulsion.
- Linear structures, like \(\text{SCN}^-\), have bond angles of 180掳 due to the alignment of all three atoms in a straight line.
- Bent shapes, such as \(\text{SO}_2\), typically have bond angles around 120掳, although this can vary slightly depending on the electron pair repulsion from lone pairs.
- Trigonal pyramidal shapes seen in \(\text{ClO}_3^-\) have bond angles of about 107掳 because the lone pair on the chlorine atom causes the bond angles to decrease from the ideal 109.5掳 found in a perfect tetrahedral shape.
