# Why cannot oxygen form 4 bonds

## Establishing structural formulas

Structural formulas are of great relevance in chemistry. In order to set up the structural formula of a molecule, we have to work through the following steps:

Step 1: Add the valence electrons of all atoms in the compound together

Step 2: Calculate the number of electron pairs: Divide the number of all valence electrons by two

Step 3:

Step 4: Distribution of the binding and free electron pairs. Important: We have to make sure that hydrogen atoms are always assigned exactly two electrons, and all other atoms eight electrons

Step 5: Finding charges within the molecule

Example 1: structural formula of water (H2O)
1. Add the valence electrons of all atoms in the compound together
An oxygen atom has six valence electrons because it is in the sixth main group. A hydrogen atom has a valence electron because it is in the first main group. Important: There are two hydrogen atoms bound in one water molecule! So for H2O we set up the formula 1 * 2 + 6 = 8. In total there are 8 valence electrons in the water molecule.
2. Calculate the number of electron pairs
The valence electrons determined in step one are now divided by two in order to know the number of electron pairs. The calculation shows that there are 4 electron pairs.
3. Calculate bonding electron pairs
To find out how many of the electron pairs are binding in a molecule, we use the formula above and get (2 * 2 + 8 * 1-8) / 2 = 2. So there are 2 bonding electron pairs.
4. Distribution of the bonding and free electron pairs.
First we write down all the atoms and distribute the binding EP. In this case there is only one option.Now we count how many electrons are missing in each case. The hydrogen atoms already have two electrons each with one bond - so we're done here.So far, the oxygen atom has only four electrons with the two bonds, so another four are missing. Therefore we assign the two lone pairs of electrons to the oxygen.
Our learning books for the chemistry Abi'21 Explanations ✔ Tasks ✔ Learning videos ✔

New!

This results in the following structural formula (without checking for charges):

5. Finding charges within the molecule
6. To determine the charges, the electron pairs are split up. We count the electrons that are assigned to the individual atoms according to this division. In the case of hydrogen atoms, there is one electron each. Since hydrogen also has a valence electron, the number of valence electrons corresponds to the number of electrons assigned in the molecule and the hydrogen atom has no charge.Six electrons are assigned to the oxygen atom. Again, there is no charge, as this number corresponds to the number of valence electrons in oxygen.
Example 2: Structural formula of sulfur dioxide (SO2)
1. Add the valence electrons of all atoms in the compound together
Sulfur and oxygen are each in the sixth main group, so that each of these atoms has six valence electrons.Important: There are two oxygen atoms bound in the molecule! To calculate the valence electrons, we insert again into the formula shown above and get: 6 + 6 * 2 = 18
2. Calculate the number of electron pairs
To find the number of electron pairs, we have to divide the valence electrons determined in the first step by two and get 18: 2 = 9 electron pairs.
3. Calculate bonding electron pairs
Now let's use the formula above. There are no hydrogen atoms, just three other atoms bound in the molecule. We calculate again: (2⋅0 + 8⋅3-18) / 2 = 3. So there are 3 binding electron pairs and 6 free electron pairs.
4. Distribution of the binding and lone pairs of electrons

We take all the atoms and distribute the binding EP. We distribute the other breaths around the chosen central atom (usually the atom that occurs once).

Our learning books for the chemistry Abi'21 Explanations ✔ Tasks ✔ Learning videos ✔

New!

Now we count again how many electrons are missing. Since the left oxygen atom has four electrons, four more are missing (two EP). The sulfur atom has six electrons, so two are missing (one EP).

The oxygen atom on the right has two electrons, six are still missing (three EP).

This results in the following structural formula (without checking for charges):

5. Finding charges within the molecule
To determine the charges, we divide the electron pairs again. We count the electrons that are assigned to the individual atoms according to this division. Six electrons are assigned to the oxygen atom on the left. Since it also has six valence electrons, there is no charge here.Five electrons are assigned to the sulfur atom. Since sulfur has six valence electrons, one less electron is assigned, which means that a single-positive charge is created here.The right oxygen atom has seven electrons assigned to it, but oxygen has six valence electrons. So there is one more negative charge than usual, which is why a single negative charge arises.

This results in the following structural formula:

Our learning books for the chemistry Abi'21 Explanations ✔ Tasks ✔ Learning videos ✔

New!