Draw Lewis structures for these ions and show which atom (or atoms) in each bears the formal charge. Q.) CH3NH3+
The Correct Answer and Explanation is:
To draw the Lewis structure for CH₃NH₃⁺ (methylammonium ion), let’s break it down step by step:
Step 1: Count the Valence Electrons
- Carbon (C) is in Group 14, so it has 4 valence electrons.
- Hydrogen (H) is in Group 1, so each hydrogen atom contributes 1 valence electron.
- Nitrogen (N) is in Group 15, so it has 5 valence electrons.
- The positive charge on the ion indicates that one electron has been lost, reducing the total valence electrons by 1.
Total valence electrons = 4 (from C) + 3×1 (from H in CH₃) + 5 (from N) + 3×1 (from H in NH₃) = 4 + 3 + 5 + 3 = 15 electrons.
- Since the ion has a positive charge, we subtract 1 electron, giving us a total of 14 valence electrons to use in the structure.
Step 2: Determine the Connectivity
- Carbon (C) will form three bonds with three hydrogen (H) atoms, making it the central atom in the CH₃ group.
- Nitrogen (N) will form three bonds with three hydrogen (H) atoms, making it the central atom in the NH₃ group.
- The CH₃ and NH₃ groups are attached to each other by a single bond between the carbon (C) and nitrogen (N) atoms.
Step 3: Draw the Structure
- Place C at the center, attached to three H atoms, and N attached to three H atoms as well.
- There will be a single bond between the C and N atoms.
- Since the ion has a positive charge, N will carry the positive formal charge because it is one electron short of its full complement (5 valence electrons). The C atom and the H atoms do not have any formal charges.
Step 4: Formal Charge Calculation
The formal charge can be calculated using the formula: Formal Charge=Valence Electrons−(Lone Pair Electrons+12×Bonding Electrons)\text{Formal Charge} = \text{Valence Electrons} – (\text{Lone Pair Electrons} + \frac{1}{2} \times \text{Bonding Electrons})Formal Charge=Valence Electrons−(Lone Pair Electrons+21×Bonding Electrons)
- For Carbon (C): Carbon forms 4 bonds (to three hydrogens and one nitrogen), and it has no lone pairs. Its formal charge is: Formal Charge on C=4−(0+4)=0\text{Formal Charge on C} = 4 – (0 + 4) = 0Formal Charge on C=4−(0+4)=0
- For Nitrogen (N): Nitrogen forms 4 bonds (three with hydrogens and one with carbon), and it has no lone pairs in this structure. Its formal charge is: Formal Charge on N=5−(0+4)=+1\text{Formal Charge on N} = 5 – (0 + 4) = +1Formal Charge on N=5−(0+4)=+1
- For Hydrogens (H): Each hydrogen has a bond with either carbon or nitrogen and does not have any lone pairs. Therefore, all hydrogens have a formal charge of 0.
Final Structure
- CH₃NH₃⁺ has a formal charge of +1 on the nitrogen atom. This positive charge results because nitrogen has one fewer electron than it would in a neutral state. The carbon and hydrogen atoms in the molecule are neutral, with no formal charge.
Summary
- Formal Charge: The N (nitrogen) atom bears the formal charge of +1 in the CH₃NH₃⁺ structure.
- The structure shows carbon (C) bonded to three hydrogens and nitrogen (N) bonded to three hydrogens, with the positive charge localized on nitrogen.
This explains the structure and formal charge distribution in the CH₃NH₃⁺ ion.
