General Chemistry

 

Transcript of the video and more practice problems below

Let’s follow the following steps to write down the Lewis structure for water, methane, and carbon dioxide.  Let’s start with the first one, water. According to this procedure, we’re first going to write the correct skeletal structure. That is, how we’re going to arrange the two hydrogens and the oxygen in the molecule. So, here are some rules. First, Hydrogens are going to be on a terminal position. What it means is that we cannot have a hydrogen in the middle of the two atoms because hydrogens can only make one bond, so this wouldn’t be proper. We are going to have something like this: oxygen connected with one hydrogen but it also says that atoms with higher bond-capacity go in the middle. Higher bond-capacity means how many bonds the atom is going to make to satisfy the octet rule. The oxygen can make two bonds to satisfy the octet rule and that’s why it’s going to be in the middle. So, the only option is to put the other hydrogen on this side, on this terminal position as well.

This is step one. We have gotten the correct skeletal structure for the water. The second step says that we need to sum the valence electrons of all the atoms. Valence electrons can easily be determined based on the periodic table: the group number tells us how many valence electrons the element has. For oxygen it is six, it is in group six which means that the oxygen has 6 electrons and hydrogen has one electron and we have two hydrogens so we have 2 electrons coming from the two hydrogens.

Of course, they have to be connected and to connect them we need to use two electrons. I am going to connect one hydrogen with oxygen and then the other hydrogen with the oxygen as well. So, we have used four electrons here to make the bonds. Next, what we’re going to do is to count how many electrons we had in total. We have eight electrons and at this step, we’re going to count how many electrons we have used. Two bonds, so we have used four electrons, -4 electrons which means that we still have four electrons and these electrons are going to be placed around the more electronegative atom to make the octet for it. If we put two lone pairs of electrons on the oxygen we can count that it has 8 electrons. Oxygen has an octet and this will be the proper Lewis structure for the molecule

This can also be represented by a line. The bonds can be represented with lines and we are still going to put these electrons here. These electrons that do not make a bond are called nonbonding electrons or lone pairs of electrons.

While these two that we have used to make the bonds are called bonding electrons. This will be the correct Lewis structure of the water.

Now, lets’ go to the second molecule. Carbon here is going to be in the middle again because hydrogens go to the terminal position. We have four hydrogens and each hydrogen, in order to make a bond with the carbon, is going to use two electrons. So, two electrons here and then two electrons for this hydrogen we have also for this hydrogen and for the 4th one. The total number of electrons that we have here: for the carbon we have four electrons because it’s group for and for the hydrogens, we have four of them, so 4 x 1 here 4 electrons coming from the hydrogens and the total number of the electrons is 8. Now we’re going to check how many electrons we have used. We made 1, 2, 3, 4 bonds and each bond is taking two electrons. So, we had 8 and we used 8 electrons. We don’t have any more electrons left which means that this is the Lewis structure of the methane. Also, the carbon has an octet here which is what it needs to follow.

Now let’s go to the next example, carbon dioxide. Carbon is going to be in the middle here because according to this rule here we are saying that the more electronegative element goes in a terminal position and oxygen is more electronegative so we are going to put them on the sides. It’s also because carbon has a higher bond-capacity which means that carbon has to make 4 bonds in order to satisfy the octet rule but we can use any of these two to put the oxygens in the right position.

We now have the skeletal structure. Lets’ sum the valence electrons. Carbon-4 electrons, oxygen-6 electrons and we have two oxygens so we have 12 electrons here and in total, we have 16 electrons from the carbon and the oxygen. They have to be connected and in order to connect them we can also show it by dots, the electrons are shown as dots. So, put this here and now we made a bond between the carbon and oxygen. When we did this, we have used four electrons. Subtract the 4 electrons and we have 12 electrons left. Let’s see what we do with these 12 electrons. So, here it says that arrange them in order to satisfy the octet rule for more electronegative element first. Which means that we are going to put these electrons on the oxygen, not on the carbon. So, 12 electrons are going to be shared between these two oxygens. We have the bonds, and 6 electrons on each oxygen. This can also be represented with lines the line and this makes it sometimes easier to see what is happening.

Now we’re going to check for the octet rule. What about the carbon? Oxygens have eight electrons to satisfy the octet rule but the carbon has two bonds and each bond is made with two electrons so what we have here is that the carbon has four electrons, so we are going to put four here. One thing to mention here is that the bonding electrons belong to both elements, they belong to the carbon and the oxygen. So, if we count the number of electrons that surround the oxygen, we circle everything that surrounds it and that’s why we have eight and we have an eight here. This is essentially the octet rule. What it says, is that some elements tend to have eight electrons around them so they tend to adopt an electron configuration of noble gases. There are many exceptions to the octet rule but at least for the second row of the elements, excluding the metals and the Boron, they usually follow the octet rule.

Eight and eight here. What do we need to do? It says that if any atom lacks an octet, and of course, this should be an atom that follows the octet rule, and carbon definitely is an atom that tries to follow the octet rule, we need to add an additional bond.

It means that we need to give, we need to share some of these electrons on the oxygen with the carbon. If I bring two electrons from the oxygen to the carbon and if I draw the resulting structure here, I will have carbon bonded to this oxygen with a double bond and the oxygen has only two lone pairs now.

The carbon has 1, 2, 3 bonds which means that these are 6 electrons and so if we represent this with dots, we’ll have oxygen, and then these are the electrons that make the double bond. So, if we circle everything that is around the carbon, it is still not an octet.

That means that we are going to take another lone pair from here and share it with the carbon, now from this oxygen. So, the carbon is now going to have a double bond and a double bond here. It’s going to have two lone pairs of electrons. Now, if you count how many electrons we have around the carbon, we have four electrons here, four electrons in the double bonds and these are the oxygens here with the lone pairs of electrons. And now, the carbon has eight electrons, so it goes without that rule now and that is what you want to have. Same, the oxygen, it has 8 electrons so all the elements now have eight electrons. And this is the last step for drawing the Lewis structure. We make sure that all the elements have an octet by making a double or a triple bond.

Practice

1.

Draw the Lewis Structures and determine the electronic and molecular geometries for the following molecules: (a) BF3, (b) CH2O, (c) HCN, (d) BeCl2, (e) CH2Cl2, (f) SOCl2, (g) SO2

answer
The answers and solutions to practice problems are available to registered users only. Click here to Register!  
Solution
The answers and solutions to practice problems are available to registered users only. Click here to Register!  

Leave a Comment