# What is the Lewis structure of C3H5+ ion?

• ArjenB
In summary, a Lewis structure is needed for a positively charged ion, and the method explained has always worked. However, this is the first time I have to give the Lewis structure of a positive ion. The valence of the C atom is 4 electrons, and 1 electron for the H atom. In order to meet the octet rule, the C atom needs 8 electrons, and the H atom needs 2 electrons. According to the method that is given, the amount of pairing electrons can be calculated by substracting the amount of valence electrons from the amount of electrons that are needed to meet the octet rule. I have learned to add electrons for a negatively charged ion, so I concluded I will have to substract electrons in
ArjenB
Hi,

For school, I have to give the Lewis structure of C3H5+ ion.

There is a method explained, which is fairly easy, and has always worked. It has also worked for ions. However, this is the first time I have to give the Lewis structure of a positive ion.The valence of the C atom is 4 electrons, and 1 electron for the H atom. In order to meet the octet rule, the C atom needs 8 electrons, and the H atom needs 2 electrons. According to the method that is given, the amount of pairing electrons can be calculated by substracting the amount of valence electrons from the amount of electrons that are needed to meet the octet rule.

I have learned to add electrons for a negatively charged ion, so I concluded I will have to substract electrons in a positive ion. So here I will have to substract 1 electron from the amount of valence electrons, due to the positive charge of 1+. Correct?

So I thought:

Valence electrons = 3C + 5H - 1 = 3 * 4 + 5 * 1 - 1 = 16 electrons
Electrons to meet octet rule = 3 * 8 + 5 * 2 = 34 electrons

So the ion is 18 electrons short. I have been told that these electrons will form bonds. So there are 18/2 = 9 electron bonds, so there should be a total of 9 bonds between all the atoms.

Then I looked up the lewis structure for this ion, and there are a total of 8 bonds in the ion, and not 9. Where have I gone wrong here?

This method has always worked so far...

ArjenB said:
Then I looked up the lewis structure for this ion, and there are a total of 8 bonds in the ion, and not 9. Where have I gone wrong here?
This compound does not obey the octet rule.

Well, of course, it's not a noble gas. Maybe I have I understood it wrong, but as far as I understood it, the electrons required to obey the octet rule, will be used to form bonds. In this case, I calculated 18 electrons would be required to obey the octet rule, which are 9 pairs, so assumably 9 bonds in the total molecule. But there aren't 9 bonds between the atoms, there are only 8. This is what I do not understand, especially looking at the fact that this method of determining the Lewis structure has always worked for me. I double checked the valence for all electrons, checked my calculations several times, but cannot find where I went wrong.

As I said, this compound (like most carbocations) does not obey the octet rule. One of your carbons will be a sextet. That means you can't use the octet rule to determine the number of bonds.

Sextet? A hexavalent carbon? Unheard of!
C3H5+ is the propene carbonium ion. It is formed (briefly) because it has two resonance forms, delocalizing the positive charge across the molecule.
1. Simplify your Lewis method: 3C x 4 v.e. = 12
count your v.e.'s 5H x 1 v.e. = 5
positive charge so subtract 1 electron -`1
16 valence electrons to work with = 8 electron pairs
2. Using electron pairs, first, connect the atoms, then disperse the remaining electrons so that the octet rule is obeyed (if possible); use double and/or triple bonds as needed. You can have lone pairs of electrons on O, S, N, P etc.
H2C=CH-CH2 (I cannot make this program format the molecules properly; there should be a p
.....⊕ positve charge on the right side carbon atom.)
if there is a charge, place it on the least electronegative atom - in this case carbon, for the reason just mentioned, and because two resonance

forms are possible, stabilizing the molecule:
H2C-CH=CH2 ↔ H2C=CH-CH2
...⊕.......⊕
these two ⊕ should be spread between the two ends. Get an organic chem book and look up RESONANCE

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1. I should clarify: Sextet refers to the fact that the Lewis structure for the compound gives an electron count of six around the carbon with the formal positive charge. It does not refer to the number of bonds to carbon (just as a full octet of electrons around carbon doesn't imply octavalent carbon).

2. Also, NB -- C3H5+ can be an allyl cation or a cyclopropyl cation.

3. Just to be really pedantic, carbon can in fact be hexavalent: consider the carboranes, or the FeMoCo core of nitrogenase.

Edit: to be really really pedantic, I should say that carbon is hexacoordinate in these situations, not necessarily hexavalent.

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## 1. What is a Lewis structure?

A Lewis structure is a diagram that represents the arrangement of atoms and valence electrons in a molecule. It is named after the American chemist Gilbert N. Lewis, who first described this type of diagram in 1916.

## 2. How do you draw a Lewis structure?

To draw a Lewis structure, first determine the total number of valence electrons in the molecule by adding together the valence electrons of each atom. Then, arrange the atoms in the molecule and connect them with single bonds. Place the remaining electrons around the atoms to satisfy the octet rule, which states that atoms tend to gain, lose, or share electrons to achieve a stable configuration with eight valence electrons.

## 3. What is the purpose of a Lewis structure?

The purpose of a Lewis structure is to show the bonding and non-bonding electrons in a molecule, giving a visual representation of its structure and properties. It also helps in predicting the shape and polarity of a molecule, which are important factors in determining its chemical and physical characteristics.

## 4. Can a Lewis structure be incorrect?

Yes, a Lewis structure can be incorrect if it does not follow the octet rule or if it does not accurately represent the arrangement of atoms in the molecule. Additionally, some molecules may have multiple valid Lewis structures, and it is important to consider all possibilities when drawing a Lewis structure.

## 5. Are there any exceptions to the octet rule?

Yes, there are a few exceptions to the octet rule. These include molecules with an odd number of valence electrons, molecules with less than an octet of valence electrons (such as boron and beryllium), and molecules with more than an octet of valence electrons (such as sulfur and phosphorus). These exceptions are mainly observed in molecules with elements from the third period or higher on the periodic table.

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