The student proposes the following Lewis structure for the nitronium (NO2+) ion: BNO2 = 04 Assign a formal charge to each atom in the student’s Lewis structure. Formal charge: Nitrogen: 0 Oxygen: 0
The Correct Answer and Explanation is:
In order to assign the formal charges for the nitronium ion (NO2+), let’s first review the concept of formal charge and how it’s calculated.
Formal Charge Formula:Formal Charge=(Valence Electrons)−(Nonbonding Electrons)−Bonding Electrons2\text{Formal Charge} = (\text{Valence Electrons}) – (\text{Nonbonding Electrons}) – \frac{\text{Bonding Electrons}}{2}Formal Charge=(Valence Electrons)−(Nonbonding Electrons)−2Bonding Electrons
Step 1: Correcting the Lewis Structure of NO2+
The nitronium ion (NO2+) has a +1 charge, which means that the ion has one fewer electron than the neutral molecule. The general structure for NO2+ is:
- The nitrogen (N) atom is at the center, connected by double bonds to two oxygen (O) atoms.
- Since the ion has a positive charge (+1), one of the oxygen atoms should have a formal positive charge to compensate for the electron deficiency.
Step 2: Formal Charge Calculations
Let’s break down the atoms in the NO2+ structure:
- Nitrogen (N):
- Nitrogen is in group 15 of the periodic table, so it has 5 valence electrons.
- In this structure, nitrogen forms two double bonds with oxygen. Each double bond contributes 2 electrons to nitrogen’s bonding.
- Since nitrogen is bonded to two oxygen atoms, it has a total of 4 bonding electrons (2 from each bond).
- Nitrogen has no lone pairs of electrons in this structure.
- Applying the formula: Formal Charge of N=5−0−42=5−0−2=+1\text{Formal Charge of N} = 5 – 0 – \frac{4}{2} = 5 – 0 – 2 = +1Formal Charge of N=5−0−24=5−0−2=+1
- So, nitrogen has a formal charge of +1.
- Oxygen (O):
- Oxygen is in group 16 of the periodic table, so it has 6 valence electrons.
- Each oxygen atom forms a double bond with nitrogen, contributing 4 bonding electrons in total (2 per bond).
- Each oxygen atom will have 2 lone pairs of electrons, contributing 4 nonbonding electrons.
- Applying the formula for each oxygen atom: Formal Charge of O=6−4−42=6−4−2=0\text{Formal Charge of O} = 6 – 4 – \frac{4}{2} = 6 – 4 – 2 = 0Formal Charge of O=6−4−24=6−4−2=0
- So, each oxygen atom has a formal charge of 0.
Final Formal Charges:
- Nitrogen: +1
- Oxygen (each): 0
The correct formal charges in this structure are:
- Nitrogen: +1
- Each Oxygen: 0
Conclusion:
The student’s initial formal charge assignment (0 for both nitrogen and oxygen) is incorrect because the positive charge on the nitrogen atom should be acknowledged in the formal charge calculation for the ion. The correct formal charge for nitrogen is +1, and for oxygen, it’s 0 in this particular structure.
