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​C2 D) Covalent Bonds
​Covalent bonds involve non-metals sharing pairs of electrons. The positively charged nuclei of the non-metals that form covalent bonds are attracted to the shared pair of electrons. This electrostatic force of attraction makes the covalent bonds extremely strong. Atoms can only share electrons in their outermost electron shells. The atoms that form covalent bonds use the covalent bonds to obtain full outer shells of electrons. Each covalent bond increases the number of electrons for both of the atoms by 1. The number of electrons that an atom wants to gain is the number of covalent bonds that the atom forms. For example, if we have an atom that wants to gain 2 electrons, the atom would form 2 covalent bonds where they share 2 of their electrons with other atoms to obtain a full outer shell of electrons. Non-metals can form single covalent bonds, double covalent bonds and triple covalent bonds.
 
Simple molecular substances are molecules that are made up of a few atoms joined together by covalent bonds. We are going to look at drawing some of these simple molecular substances in this section.
Fluorine

Fluorine travels around in the form of F2 (it is a diatomic molecule; travels around in pairs). The tile in the periodic table for fluorine is shown below along with the number of protons, neutrons and electrons.

Fluorine has 9 electrons; 2 of these electrons will be in the first shell and 7 will be in the second shell. The two separate fluorine atoms are shown below. The electrons for the fluorine on the left are dots, and the electrons for the fluorine on the right are crosses.
​The second electron shell takes a maximum of 8 electrons. Currently, both of the fluorine atoms have 7 electrons in their outer shells, which means that they are 1 off having a full outer shell; they want to gain 1 electron. The number of electrons that they want to gain is the number of electrons that they share; this means that each of the fluorine atoms shares 1 electron. They share these electrons in the overlapping part of the two outer shells; in the overlapping part, there will be 1 dot and 1 cross. Both of the fluorine atoms had 7 electrons in their outer shells, and both of them have put 1 electron into the sharing part. This means that they both have 6 electrons left that will stay on their own outer shells. The outcome for the bonding is shown below.
We can now count to see if both of the atoms are happy – do both of the atoms have full outer shells? When we count the electrons, we count all of the electrons in the outermost shell including the ones in the sharing part. When we count the electrons, we can see that both of the fluorine atoms have 8 electrons in their outermost shell; their outer shells are full.

As there is only 1 pair of shared electrons, the fluorine atoms have formed a single covalent bond. We represent a single covalent bond with a single line (–). The displayed formula for F2 is shown below.

Oxygen

Oxygen travels around in the form of O2 (it is a diatomic molecule).  The tile in the periodic table for oxygen is shown below along with the number of protons, neutrons and electrons.

Oxygen has 8 electrons; 2 of these electrons will be on the first shell and 6 will be on the second shell. The two sperate oxygen atoms are shown below. The electrons for the oxygen on the left are dots, and the electrons for the oxygen on the right are crosses.
The second electron shell takes a maximum of 8 electrons. Currently, both of the oxygen atoms have 6 in their second/ outer shell, which means that they are 2 electrons off having a full outer shell; they want to gain 2 electrons. The number of electrons that they want to gain is the number of electrons that they share; as each of the oxygen atoms want to gain 2 electrons, they both put 2 electrons into the sharing part (the overlapping part of the outer shells). This means that in the sharing part, there will be 2 dots and 2 crosses. Both of the oxygen atoms had 6 electrons in their outer shells, and both of them have put 2 electrons into the sharing part. This means that they both have 4 electrons left that will stay on their own outer shells.

We can check that both of the oxygen atoms are happy by counting the electrons in the outer shells of the oxygen atoms to see if the shells are full. The outer shell for the oxygen atoms is the second shell, which takes a maximum of 8 electrons. When we count the electrons on the outer shells for both of the oxygen atoms, we see that they both have 8 electrons and are therefore happy.

 

The oxygens shared 2 pairs of electrons and this means that we have a double covalent bond. We represent a double covalent bond with a double line (=). The displayed formula for O2 is shown below.

Nitrogen

Sometimes you will be asked to just draw the outmost electron shells for the atoms rather than all of the electron shells. This is because the inner electron shells can make the diagrams messy/ less clear. We are going to have a look at an example where this is the case.

 

Nitrogen travels around in the form of N2 (it is a diatomic molecule). The tile in the periodic table for nitrogen is shown below along with the number of protons, neutrons and electrons.

​Nitrogen has 7 electrons; 2 of these electrons will be in the first shell and 5 will be in the second shell. The two sperate nitrogen atoms are shown below. The electrons for the nitrogen on the left are dots, and the electrons for the nitrogen on the right are crosses.
The second electron shell takes a maximum of 8 electrons. Currently, both of the nitrogen atoms have 5 electrons in their outer shell, which means that they are 3 electrons off having a full outer shell; they want to gain 3 electrons. The number of electrons that they want to gain is the number of electrons that they share; this means that as each of the nitrogen atoms want to gain 3 electrons, they both put 3 electrons into the sharing part (the overlapping part of the shells). Both of the nitrogen atoms had 5 electrons in their outer shell and both of them have put 3 into the sharing part. This means that they both have 2 electrons left that will stay on their own outer shells. I am going to just show the outermost shells for the nitrogen atoms and not show the first shell. The diagram is shown below.

We can check that both of the nitrogen atoms are happy by counting the electrons on each of their outer shells. Their outer shells are the second shell which takes a maximum of 8 electrons. When we count electrons, we see that they both have 8 electrons in their outer shells and are therefore happy.

 

The nitrogen atoms share 3 pairs of electrons and this means that we have a triple covalent bond. We represent a triple covalent bond with 3 lines (≡). The displayed formula for N2 is shown below.

Hydrogen Chloride
All of the previous examples have involved the same elements. We are now going to have a look at a few examples where the atoms involved are different elements. The first example that we are going to look at is hydrogen chloride (HCl). The tiles in the periodic table and the number of protons, neutrons and electrons for hydrogen and chlorine are shown below.
The electronic structure for hydrogen and chlorine are shown below.
The outer shell for hydrogen is its first shell and the first shell takes a maximum of 2 electrons. Currently, hydrogen has 1 electron in its first shell, which means that it needs to gain 1 electron in order to have a full outer shell.
 
The outer shell for chlorine is its third shell and the third shell takes a maximum of 8 electrons. Currently, there are 7 electrons in its outer shell, which means that it needs to gain 1 electron in order to have a full outer shell.
 
As both of these atoms are 1 off having a full outer shell, they share one pair of electrons; both of the atoms put 1 electron into the sharing part. The diagram for the bonding is shown below.  
If we count the electrons in the outer shells for both of the atoms, we can see that both of the atoms have full outer shells and are therefore happy; hydrogen has 2 electrons in its outer (first) shell so is happy, and chlorine has 8 electrons in its outer (third) shell so is also happy.
 
The hydrogen and chlorine shared 1 pair of electrons, which means that they formed 1 covalent bond. The displayed formula for HCl is shown below.
Methane

The next example that we are going to look at is methane, which is CH4. Methane is made out of 1 carbon atom and 4 hydrogen atoms. The tiles in the periodic table and the number of protons, neutrons and electrons for hydrogen and carbon are shown below (I have only drawn 1 hydrogen atom and not all 4).

​Carbon has 4 electrons in its outer shell, which is the second shell. The second shell takes a maximum of 8 electrons, and this means that carbon needs to gain 4 electrons to have a full outer shell.

Each of the hydrogens has 1 electron in its outer shell, which is the first shell. The first shell can take a maximum of 2 electrons and this means that each of the hydrogen atoms need to gain 1 electron in order to have a full outer shell.
 
So, carbon needs to gain 4 electrons and all of the 4 hydrogens need to gain 1 electron. All of the atoms can obtain full outer shells by all of the hydrogen atoms sharing 1 electron with the carbon, and carbon sharing 1 electron with each of the 4 hydrogen atoms. The diagram of the bonding for methane with only the outer shells drawn is shown below.

Let’s now check that all of the atoms involved have full outer shells. The outer shell for all of the hydrogen atoms is the first shell, which takes a maximum of 2 electrons, and all 4 of the hydrogen atoms have 2 electrons in their outer shells. The outer shell for carbon is the second shell, which takes a maximum of 8 electrons, and carbon on our diagram does have 8 electrons in its outer shell. All of the 5 atoms involved have full outer shells and are therefore happy.

 

All of the bonds in the above diagram are single covalent bonds as there is only 1 pair of shared electrons in each of the overlapping parts of the shells. The displayed formula for CH4 is shown below.

Carbon Dioxide

The final example that we are going to look at is carbon dioxide, which is CO­2. There is 1 carbon atom and 2 oxygen atoms in carbon dioxide. The tiles in the periodic table and the number of protons, neutrons and electrons for carbon and oxygen are shown below.

Carbon has 4 electrons in its outer shell, which is the second shell. The second shell takes a maximum of 8 electrons, and this means that carbon needs to gain 4 electrons to have a full outer shell. Both of the oxygens have 6 electrons in their outer shell, which is also the second shell; this means that both of the oxygens are 2 electrons off having a full outer shell.
 
So, carbon needs to gain 4 electrons and both of the oxygen atoms need to gain 2 electrons. All of the atoms can obtain full outer shells by carbon sharing 2 electrons with each of the oxygens, and each of the oxygens sharing 2 electrons with carbon. The diagram of the bonding for carbon dioxide with only the outer shells drawn is shown below.

Let’s now check that all of the atoms involved have full outer shells. The outer shell for all of the atoms is the second shell, which takes a maximum of 8 electrons. When we count the electrons in each of the outer shells, we see that they all have 8 electrons, thus meaning that the outer shells are full and the atoms are happy.

 

For both of the bonds in carbon dioxide, there are 2 pairs of shared electrons. This means that both of the bonds are double covalent bonds; we represent this with a = in the displayed formula. The displayed formula for CO2 is shown below.

The Positive & Negatives
We are now going to have a look at the positives and negatives of dot & cross diagrams (dot & cross diagrams are the diagrams that we have looked at in this section). A positive of dot & cross diagrams is that they are good at showing which atoms the electrons come from. A negative of dot & cross diagrams is that they do not show the relative sizes of the atoms.