# Lewis Structure of the Guanidinium Ion

In a problem, I was asked to find the Lewis structure of the guanidinium ion $$\ce{C(NH2)3}^{+1}$$. I followed the following steps which led me to an incorrect structure and I was hoping someone could help me find my mistake.

1. The total number of valence electrons is given by $$4+3(5+2(1)) +1 = 26.$$
2. The total number of electrons with each atom having an octet is $$8+3(8+2(2)) = 44.$$
3. The number of bonding electrons is thus $$44-26 = 18.$$
4. I drew the following skeletal structure which includes $$9$$ bonds (corresponding to $$18$$ bonding electrons):

1. I have $$26 - 18 = 8$$ non-bonding electrons remaining, which I assigned as lone pairs to the three nitrogens and carbon:

However, this structure is clearly incorrect because carbon rarely forms three bonds. The correct structure (including resonance) is

• There's no lone pair on carbon... Commented Jun 6, 2023 at 20:40
• Commented Jun 6, 2023 at 22:18
• Hint: Start with guanidine $\ce{HN=C(NH2)2}$ Commented Jun 7, 2023 at 8:49

$$4+[3×(5+2)]\color{red}{+}1=26$$ 🚫
$$4+[3×(5+2)]\color{blue}{-}1=24$$ ✔️