There are many forms of carbon but the two best-known crystalline forms are graphite and diamond. The existence of different crystalline forms of an element is known as allotropy. Graphite consists of planar sheets of hexagonal units of carbon atoms with each carbon bonded to another three carbon atoms. This has found application as a lubricant, as one plane can slide easily across the adjacent plane. Diamond is made up of tetrahedral units of carbon, where a carbon atom is surrounded by four other atoms of carbon. Charcoals, a common form of carbon, do not have a crystalline form.

Tutorial 1

How can you tell the age of a plant using carbon? Answer

The density of graphite is 2.22 gm cm‾� while that for diamond is 3.51 gm cm‾�. If we applied 15,000 atmosphere of pressure to graphite at a temperature of 300�C, it would be at conversion equilibrium with diamond. It would change into diamond very slower. In nature it took thousands of years. At the moment we can only produce industry diamonds (about 0.1 carat big) using this technique.

The electrons in graphite can move along the plane and so graphite can conduct electricity. It is used as contacts in electromagnetic coils. The electrons in diamond are held firmly in the carbon-carbon bond and so is non-conducting.

Carbon is the most important element in the world today. All materials present in living things are made up compounds containing carbon. Carbohydrates, cellulose, proteins, etc. So much so a special branch of chemistry, known as Organic Chemistry, was created to study the chemistry of carbon compounds. What is left is known as Inorganic Chemistry. So far we have been doing an overview of chemistry. Commonly referred to as General Chemistry. The other broad area of Chemistry is known as Physical Chemistry. It deals more with the physics of chemistry. I will continue with General Chemistry for the time being.

Carbon is unique in the sense that it has four valence electrons, half way between two stable electron shells (He and Ne). It can give away four electrons to achieve the helium K-shell or acquire four electrons to reach the neon L-shell. The first IP is 1086.2 kJ/mol and the second 2352 kJ/mol. Extracting four electrons from carbon requires too much energy so carbon do not form metallic salts - chloride, sulphate, nitrate, carbonate - like the metallic elements. For example there is no carbon chloride.

Carbon reacts by sharing its four electrons to form covalent bonds. The electron configuration for carbon is [He] : 2s�, 2p�.  Invoking the hybridization of the atomic orbitals we can obtain the following electron configurations:

C	[He] :	four sp�
C	[He] :	three sp�, 2p�
C	[He] :	two sp�, 2p�

sp�

Methane

When the carbon atom reacts via this electron configuration, the molecule formed will have a tetrahedral structure. Example methane, CH₄. Mathematics recommends an angle of about 109� between the bonds.

Tutorial 2

• Explain the bonding between carbon and the other atoms in chloroform (CHCl₃). Comment on the relative bond lengths and bond angles in the molecules.
• Derive the structure of a carbonate (CO₃‾�) ion. Comment on the relative bond lengths and bond angles in the molecules.      Answer

sp�

In the case of phosgene, C(=O)Cl₂, the carbon will react with the three atoms via the sp� valence electrons. The carbon atom forms σ-bonds with the three atoms. The remaining p-orbital in carbon then interacts with the p-orbital of the oxygen atom to form a π-bond. The structure of phosgene is planar.

If all three participating atoms were identical they would form identical bonds and the angle would be 360�/3 = 120�. The carbon-oxygen atoms are bonded by a σ-bond plus a π-bond. So the electron density between these two atoms will be higher than that between the carbon-chloride atoms, thus it needed more space. Consequently the Cl-C-O angle is more than 120� while that for Cl-C-Cl is less than 120�. (Bonds are where electrons stay, and so bonds will repel each other electrostatically.)

Tutorial 3

Using the sp� atomic hybrid for the carbon atom construct the structure of graphic and explain how the electrons can move along the plane.      Answer

This reaction is very important as it allows the oceans to act as a sink to absorb carbon dioxide from the atmosphere. This is very important, as too much carbon dioxide will prevent heat from escaping into space, causing the earth to become warmer. (Greenhouse Gases)

We can increase the solubility of carbon dioxide in water by applying pressuring. This is what happens in fizzy drinks. The sharp taste is due to the carbonic acid.

We have already seen that when you bubble carbon dioxide into a calcium hydroxide solution fine particles of calcium carbonate will be precipitating out. This is often used as a qualitative test for carbon dioxide.

Tutorial 4

Currently carbon dioxide is generated in large quantity by the burning of fossil fuel; coal and gasoline. Plants would recycle the carbon dioxide by converting them into hydrocarbons and oxygen. However carbon dioxide is only slightly soluble in water. Suggest a possible explanation for the ocean to keep absorbing large quantities of carbon dioxide.      Answer

CARBON MONOXIDE

The other form of oxide is carbon monoxide. It also react via sp hybrid atomic orbitals.

In carbon monoxide, CO, one of the sp orbitals has no electron and the other has two electrons. The empty sp orbital of the carbon interacts with a filled sp orbital of the oxygen atom to form a dative σ-bond, (that is the oxygen contributes both the electrons needed to form the σ-bond). The adjacent p-orbitals of both the atoms then overlap with the two p-orbitals of the carbon atom to form a δ-bond. So the carbon and oxygen is held together by a (dative) σ-bond and a δ-bond.

	�
The red arrow in the product represents a σ−bond with oxygen contributing the entire two electrons

The carbon monoxide has a lone pair of electrons along the x-axis, and thus behaves like a Lewis base. (A Lewis acid look for a lone pair of electrons, a Lewis base can contribute a lone pair of electrons.) So when carbon monoxide gets into our blood system it can react with the iron cations in the hemoglobin blocking the iron from carrying oxygen. This can result in death.

Similarly carbon monoxide can also react with other metal cations like nickel to give nickel tetracarbonyl, Ni(CO)₄. Nickel reacts with carbon monoxide at room temperature and pressure to give nickel tetracarbonyl. On warming nickel tetracarbonyl the reaction is reversed regenerating the nickel. This technology is used to purify nickel. It must be noted that nickel tetracarbonyl vapour is highly toxic, and is explosive when mixed with air.

Tutorial 5

Both oxygen and carbon in carbon monoxide have a lone pair of electrons, which one is more likely to share the electrons? Why? (Hint: electronegativity).      Answer