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Magazine Chapter 22: Problem 57
Write the Lewis structure for each of the following species, and describe its geometry: (a) \(\mathrm{HNO}_{2}\), (b) \(\mathrm{N}_{3}^{-}\), (c) \(\mathrm{N}_{2} \mathrm{H}_{5}{ }^{+},(\mathrm{d}) \mathrm{NO}_{3}^{-}\).
Short Answer
Expert verified
(a) HNO鈧� has a bent molecular geometry with the following Lewis structure:
O=N-O
||
H
(b) N鈧冣伝 is linear for the central nitrogen, and triangular-planar for terminal nitrogen atoms with this Lewis structure:
:N-N鈮:
||
(c) N鈧侶鈧呪伜 has a tetrahedral geometry for the first nitrogen and a trigonal pyramidal geometry for the second nitrogen. The Lewis structure is:
H
|
HN-NH
| |
H H
(d) NO鈧冣伝 is trigonal planar in geometry with the following Lewis structure:
O
||
N-O
|
O
||
Step by step solution
01
Identify the central atom
The central atom in HNO鈧� is nitrogen (N) since it is the least electronegative atom.
02
Calculate total number of valence electrons
The total number of valence electrons in HNO鈧� is:
1 (H) + 5 (N) + 6脳2 (O) = 18 electrons
03
Arrange atoms and distribute electrons in bonds
Place the nitrogen atom in the center and connect it to the other atoms:
04
Add lone pairs of electrons to the outer atoms
Oxygen needs to have 6 valence electrons, so we'll add 2 lone pairs on each oxygen atom, and 1 lone pair on the nitrogen atom for a total of 18 electrons:
O=N-O
||
H
05
Check the formal charge
After calculating the formal charge of each atom, we find that the formal charges are as follows:
O: -1
N: +1
O: 0
H: 0
Since the sum of the formal charges is zero, we don't need to adjust the Lewis structure.
06
Determine the electron group arrangement and molecular geometry
The electron group arrangement around the nitrogen atom is trigonal planar, and since there are two bonding groups and one lone pair, the molecular geometry is bent.
**(b) N鈧冣伝**
07
Identify the central atom
In N鈧冣伝, there is no central atom as all nitrogen atoms are linked in a chain.
08
Calculate total number of valence electrons
The total number of valence electrons in N鈧冣伝 is:
5脳3 (N) + 1 (additional electron) = 16 electrons
09
Arrange atoms and distribute electrons in bonds
First, form a single bond between each nitrogen atom:
NNN
10
Add lone pairs of electrons to the outer atoms
Adding lone pairs of electrons, we have:
:N-N鈮:
||
11
Check the formal charge
The sum of the formal charges is -1, which matches the overall charge of N鈧冣伝.
12
Determine the electron group arrangement and molecular geometry
The electron group arrangement of the central nitrogen is linear, and since there is one bonding group and no lone pairs, the molecular geometry is also linear. The geometries for terminal nitrogen atoms are both triangular-planar.
**(c) N鈧侶鈧呪伜**
13
Identify the central atom
There are two nitrogen atoms in this molecule. We can select any one as the central atom, and the other nitrogen will be treated as an outer atom.
14
Calculate total number of valence electrons
The total number of valence electrons in N鈧侶鈧呪伜 is:
5脳2 (N) + 1脳5 (H) - 1 (positive charge on the compound) = 14 electrons
15
Arrange atoms and distribute electrons in bonds
Arrange the atoms and distribute the electrons:
16
Add lone pairs of electrons to the outer atoms
Add the valence electrons by adding a lone pair to one of the nitrogen atoms:
H
|
HN-NH
| |
H H
17
Check the formal charge
The sum of the formal charges is +1, which is equal to the overall charge of the species.
18
Determine the electron group arrangement and molecular geometry
The geometry around the first nitrogen is tetrahedral, while the geometry around the second nitrogen is trigonal pyramidal.
**(d) NO鈧冣伝**
19
Identify the central atom
The central atom in NO鈧冣伝 is nitrogen (N) since it is the least electronegative atom.
20
Calculate total number of valence electrons
The total number of valence electrons in NO鈧冣伝 is:
5 (N) + 6脳3 (O) + 1 (additional electron) = 24 electrons
21
Arrange atoms and distribute electrons in bonds
Arrange the atoms and draw a single bond between each oxygen atom and the nitrogen atom:
22
Add lone pairs of electrons to the outer atoms
Add lone pairs of electrons on each oxygen atom:
O
||
N-O
|
O
||
23
Check the formal charge
The sum of the formal charges is -1, which is equal to the overall charge of the species.
24
Determine the electron group arrangement and molecular geometry
The electron group arrangement around the nitrogen atom is trigonal planar, and since there are three bonding groups and no lone pairs, the molecular geometry is also trigonal planar.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Molecular Geometry
Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. This arrangement helps us understand the shape of the molecule, which can influence the molecule's physical and chemical properties. For instance, in the case of
- HNO鈧� (Nitrous acid), the molecular geometry is bent because there is one lone pair on the nitrogen atom affecting its shape.
- N鈧冣伝 (Azide ion) has a linear geometry as there are no lone pairs and the atoms align in a straight line.
- N鈧侶鈧呪伜 (Hydrazine protonated form), the first nitrogen has a tetrahedral shape, while the second nitrogen has a trigonal pyramidal shape due to the presence of lone pairs.
- NO鈧冣伝 (Nitrate ion), the molecular shape is trigonal planar as it contains three bonding groups with no lone pairs on the central nitrogen.
Formal Charge
Formal charge is a theoretical charge calculated for each atom in a molecule. It helps in identifying the most stable Lewis structure of a molecule. To compute the formal charge, use the following formula:\[ \text{Formal Charge} = \text{Valence Electrons} - \text{Non-bonding Electrons} - \frac{\text{Bonding Electrons}}{2} \]In Lewis structures:
- Each atom's formal charge must be as close to zero as possible, which indicates a more stable structure.
- For HNO鈧�, the nitrogen atom has a +1 charge, and one oxygen has a -1 charge, balancing each other out.
- The N鈧冣伝 ion shows different charges distributed across the nitrogen atoms to achieve the overall -1 charge.
- In N鈧侶鈧呪伜, the formal charges are spread to add up to the total charge of +1.
- For NO鈧冣伝, even though one oxygen has a -1 charge, the other etc. even them out reaching an overall -1 charge, which matches the molecule's net charge.
Valence Electrons
Valence electrons are the outermost electrons of an atom, crucial in determining how atoms bond with each other. They play a pivotal role in forming chemical bonds and the structure of molecules. Here's a breakdown:
- In HNO鈧�, adding up the valence electrons from H (1), N (5), and O (6 each) gives a total of 18 valence electrons.
- N鈧冣伝 presents a case where the total number of valence electrons is increased by one due to the negative charge, resulting in 16 electrons for the entire ion.
- N鈧侶鈧呪伜 has a total of 14 valence electrons because the positive charge reduces the electron count by one.
- Finally, NO鈧冣伝 incorporates an extra electron due to its negative charge, totalling 24 valence electrons.
Electron Group Arrangement
Electron group arrangement refers to the three-dimensional distribution of electron groups around a central atom. An electron group can be a lone pair or a bond, single or multiple. The arrangement helps predict the overall shape of the molecule.
- For HNO鈧�, there is a trigonal planar electron group arrangement around the nitrogen due to two oxygen atoms and one lone pair.
- N鈧冣伝 is primarily linear as it consists of bonding pairs evenly distributed along the nitrogen atoms.
- In N鈧侶鈧呪伜, the first nitrogen exhibits a tetrahedral arrangement while the second exhibits a trigonal pyramidal shape.
- NO鈧冣伝 features a planar arrangement due to three equivalent bonding groups surrounding the central nitrogen.
