C3.5 – Molecules and Covalent Bonds

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1. State that non-metallic elements form non-ionic compounds using a different type of bonding called covalent bonding.

In ionic bonding, the atoms lost or gained electrons. In covalent bonding, atoms share their electrons. And this happens when two non-metals react.

 

2. Draw dot-and-cross diagrams to represent the sharing of electron pairs to form single covalent bonds in simple molecules, exemplified by (but not restricted to) H2, Cl2, H2O, CH4 and HCl.

Let’s take two hydrogen atoms reacting. Both have a configuration of just 1. So, in order to be stable both require an extra atom. So they share their one electron with the other, so both atoms now have two electrons- they are stable. This is a single covalent bond. Only one pair of electrons is shared between the two atoms. A hydrogen molecule is formed denoted as H_2.

 

Another example: let’s take the reaction between hydrogen and oxygen. Hydrogen has 1 valence electron and oxygen has (configuration: 2, 6) 6 valence electrons. Oxygen needs two electrons to be stable and hydrogen needs one. This is tricky because there is no way both can become stable. So, a second hydrogen atom is bought into play. Now there are two hydrogen atoms that need one electron each and a oxygen atoms that needs two electrons. So the hydrogen atoms and oxygen atom shares 2 pairs of electrons with each other. This is also a single covalent bond, since the two bonds created are not within the same atoms! Now you have H₂O (water!) as the product.

(Try drawing the covalent bond diagrams for reactions between (i) two Chlorine atoms, (ii) between carbon and hydrogen and (iii) between hydrogen and chlorine. Check your answers with the ones given at the bottom of this page. Hint: they all form single covalent bonds)

 

3. Describe the differences in volatility, solubility and electrical conductivity between ionic and covalent compounds.

 

Covalent bonds within molecules are strong. However the forces between the molecules themselves (intermolecular forces) are weak.

• As we learned in the previous topic, ionic structures have high melting and boiling points because of their strong forces of attractions between oppositely charged ions.
• Covalent bonds, however have lower melting and boiling points since their inter-molecular forces are weak (there are some exceptions however; one being silicon dioxide).
• Ionic compounds are soluble in water but insoluble in organic compounds (compounds that contain carbon). They are soluble in water because the water is able to separate the ions and keep them in water.
• Covalent compounds are insoluble in water with some exceptions that include sugar and amino acids.
• Ionic compounds conduct electricity only when molten or dissolved in water. This is because the ions are free to move in a liquid state. A solid ionic compound will not thus conduct electricity.
• Covalent compounds do not conduct electricity because they don’t have any ions i.e. charged particles. Their molecules are uncharged. However some covalent compounds react with water to form ions and these can conduct electricity.

 

4. Draw dot-and-cross diagrams to represent the multiple bonding in N2, C2H4 and CO2

The covalent bonds we’ve seen above are all single covalent bonds. Now let’s look at multiple covalent bonds.

 

The reaction between carbon and oxygen (both non-metals) produces a double covalent bond. Oxygen has a 6 valence electrons and carbon has 4 valence electrons. Oxygen needs 2 more electrons to be stable and carbon needs 4 more electrons to be stable. So first we need to add one more oxygen atom, so that there are now a total of 4 oxygen valence electrons as well. Now, four electrons are shared between the oxygen atoms and carbon atoms, as shown in the diagram. Each oxygen atom shares two pairs of electrons with the carbon atom. Now all atoms are stable. This is a double covalent bond, since two pairs of electrons are being shared between both atoms. The resulting compound is carbon-dioxide (CO₂)
When two nitrogen atoms react, a triple covalent bond is created. Nitrogen has a 5 valence electrons. Both nitrogen atoms will thus need 3 electrons to become stable. So they share 3 pairs of electrons to become stable. This is an example of a triple covalent bond. This is a nitrogen molecule (N₂)

 

(Try drawing the diagram of the reaction between between (iv)two Carbon atoms and Hydrogen and check your answers below. Hint: multiple atoms of  hydrogen will be required and the resulting bond is double covalent)

 

Answers:

(i) two Chlorine atoms:
(Cl₂)

(ii) Carbon and hydrogen atoms:
(CH₂)

(iii) Hydrogen and Chlorine atoms:
(HCl)

(iv) Two Carbon atoms and hydrogen:
(C₂H4)

 

 

Notes submitted by Lintha

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