The Elements : Carbon


Name Symbol Atomic Number Electronic Structure Sublimation Point (°C)
 Carbon  C 6 2 - 4 3642

Carbon is from the Latin for charcoal. It is element No 6. Carbon is the most important element in terms of life on this planet.

The symbol is C and the name comes from the Latin for charcoal. This is the first element so far that has been known since ancient times.

Carbon has six electrons. Two are in the inner shell and four are in the outer shell. The second shell of an atom can hold up to eight electrons so Carbon is half way to filling this second shell. This makes Carbon very versatile chemically.

Carbon forms two compounds with Oxygen: Carbon Monoxide (CO) and Carbon Dioxide (CO2). These show how the properties of substances vary with just a slight change in the number of atoms in the molecule.

Carbon Monoxide is a very poisonous gas. In most animals, Oxygen is carried around the body by attaching to molecules of Haemoglobin in the blood. This attachment is loose and the Oxygen is released easily. Carbon Monoxide also attaches to Haemoglobin but this link is very strong. The Haemoglobin is unavailable to carry Oxygen and the animal dies from lack of Oxygen. Carbon Monoxide is very insoluble in water.

In comparison, Carbon Dioxide is a non toxic gas. It is a product of respiration and makes up 1% of the Earth's atmosphere. It absorbs infra red radiation which is emitted by the Earth while being transparent to the sunlight which warms the Earth. This causes the temperature to rise in a phenomenon called the Greenhouse Effect.

Unlike Carbon Monoxide, Carbon Dioxide is very soluble in water, producing a weak acid solution called Carbonic Acid (H2CO3). This is used in fizzy drinks. This acid is produced in rain and dissolves minerals to produce Carbonates. The most common carbonate is Calcium Carbonate which occurs in several forms like chalk, lime stone and marble as well as egg shells and coral.

Carbon Dioxide makes up 1% of the Earth's atmosphere. It is absorbed by green plants and converted to carbohydrates under the action of sunlight. Animals produce the gas after taking in oxygen for respiration.

Carbon also forms binary compounds with metals. Examples are Tungsten Carbide (with the unfortunate formula WC), a hard substance used as an abrasive, and Calcium Carbide (Ca2C), also called Lime. Lime reacts with water giving out light. It used to be used in theatres and is the origin of the phrase "in the limelight".

Carbon is the most important element in terms of life on this planet.

The Carbon atom is remarkable in that it can combine with itself and with other atoms in a multitude of ways. This unique property means that Carbon can form a large variety of molecules with itself and other atoms. These molecules can consist of chains of carbon atoms or even rings. For example, Carbon forms millions of compounds just with Hydrogen. The simplest is Methane (CH4). There is a whole series of compounds with more carbon atoms: Ethane (C2H6), Propane (C3H8), Butane (C4H10), etc. These compounds are called Hydrocarbons. The simpler ones are gases, the more complex ones are liquids (eg, Octane C8H18 is found in petrol / gasoline). Above this you get the waxes.

Other examples are Benzene (C6H6), a solvent with a ring shaped molecule, Naphthalene (C10H8), the smell of moth balls, and Acetylene (C2H2), a gas used for welding. Polythene is a hydrocarbon consisting of chains that are millions of atoms long. Hydrocarbons power our technological civilisation.

Carbon can combine with Oxygen as well as Hydrogen. This produces several series of compounds depending on exact location and number of Oxygen atoms in the molecule. These compounds are called Carbohydrates and include the following families of substances:

Alcohols like Methanol (CH3OH in metholated spirits), Ethanol (C2H5OH, in wine, beer and spirits), etc.

Ethers like Dimethyl Ether (CH3-O-CH3, a gas used as an anaesthetic, etc.

Aldehydes like Formaldehyde (CH2O - used to preserve forensic samples), Acetaldehyde (CH3H2O), etc.

Fatty Acids like Formic Acid (HCO2H in ant bites and bee stings), Acetic Acid (CH3CO2H in vinegar), etc.

Ketones like Acetone (CH3COCH3 in nail varnish remover), etc.

Other carbohydrates include Citric Acid, Ascorbic Acid (Vitamin C), sugars and starches. Table sugar has the formula C12H22O11, a molecule with 12 carbon atoms, 22 hydrogen atoms and 11 oxygen atoms. Fats are also carbohydrates. Esters are used in perfumes. Carbolic Acid used in some soaps has the formula C6H5OH.

The number of these compounds is literally infinite. The numbers are increased by a phenomenon called Isomerism which I will now explain. Let is take two atoms of carbon, six atoms of hydrogen and one of oxygen. This could be written as C2H6O. How many different molecules can we make out of these atoms?

The answer is two.

We can arrange it so that the two carbon atoms are next to each other. The first carbon atom has three hydrogen atoms attached to it. The middle carbon atom has two hydrogen and the oxygen. The oxygen atom holds on to one hydrogen. This molecule can be written as CH3CH2OH. This substance is called Ethanol and is drunk by people in bars and pubs. This is the alcohol in drinks.

We can also arrange our atoms so that the oxygen atom is in the middle. This atom then holds on to two carbon atoms, each of these with three hydrogen atoms attached. We can write this molecule as CH3-O-CH3 (I have used dashes for clarity). This has the same number of atoms as our alcohol above but it is a completely different substance as the atoms are arranged differently. This chemical is called Dimethyl Ether. Unlike ethanol, this substance is a gas. It is used as an anaesthetic.

Isomers are two molecules with the same number of atoms but with different structures. Molecules with two carbon atoms can have two isomers but the number of isomers increases rapidly as we use more carbon atoms.

The chemistry of Carbon is called Organic Chemistry. There are more carbon compounds than compounds of all the other elements put together.

Organic compounds can also contain Nitrogen.

Amides (like Urea), Nicotine, Cyanide and the Amino Acids (the building blocks of proteins) are Organic compounds containing Nitrogen. Another is Tri Nitro Toluene (TNT, an explosive). The addition of other elements like Sulphur or Phosphorous, give the possibility of more compounds. Haemoglobin consists of a large protein molecule containing an Iron atom in the centre. Chlorophyll, used by plants to convert sunlight to sugars, is another large protein molecule containing a Magnesium atom.

On Earth all life is based on the chemistry of Carbon. Experiments have been performed where simple compounds are subjected to ultra violet light and combine to form simple organic compounds. Meteorites (rocks floating in space) are found to contain organic compounds in traces. Also, gases in interstellar space are found to contain organic compounds. Saturn's Moon, Titan is known to have Organic molecules in its atmosphere. Carbon appears to be the only atom capable of forming such complexity in its chemistry.

The question of whether Extraterrestrial Life exists is a fascinating one.

In elemental form, Carbon has two common forms.

If the atoms are arranged in layers, it forms graphite. This is a soft gray substance that conducts electricity. It is used in pencil lead, for making non-metalic electrodes, in the control of nuclear reactions and as an industrial lubricant.

If the atoms are arranged symmetrically, it forms diamond. This is a colourless crystal which is the hardest substance known and does not conduct electricity or react with chemicals easilly. It is used in jewelry and in drilling.

There is also a non-crystaline (amorphous) form present in coal, charcoal and soot. This form is black.

Three totally different substances, made up of different arrangements of Carbon atoms. These forms are called allotropes. Many elements occur as different allotropes.

Carbon has the highest melting point of any solid element (3600°C). In fact, under Earth conditions, carbon changes directly from solid to gas. This process is called sublimation.

Carbon can be alloyed with Iron to form the much tougher and longer lasting steel.

We have looked at the physical properties of Carbon (its colour, melting point). We have looked at its unique and dazzling chemical properties. We will end by looking at its nuclear properties.

The nucleus of the Carbon atom contains 6 positively charged protons and six uncharged neutrons. This makes twelve particles in the nucleus. This is called Carbon-12. 99% of all carbon atoms are Carbon-12.

1% of carbon atoms have an extra neutron. This gives us 6 protons and 7 neutrons in the nucleus. This atom is called Carbon-13. Carbon-13 has the same physical and chemical properties as Carbon-12. Both are Carbon. The chemistry of an atom is determined only by its electrons which orbit about the nucleus. When atoms differ only in the structure of their nuclei, they are called Isotopes. Carbon is said to have two stable isotopes: Carbon-12 and Carbon-13.

It is possible to have a Carbon atom with yet one more neutron (giving 6 protons and 8 neutrons). This, unsurprisingly, goes by the name of Carbon-14. A tiny amount of this type of Carbon atom is present in the atmosphere and in all living things. The problem with the Carbon-14 nucleus is that it is unstable. It has too many neutrons. After a while it will shoot out a particle and change into another type of nucleus. This phenomenon is called Radioactivity.

Radioactive nuclei decay at a known rate that is unaffected by temperature or pressure or whether the atom is on its own or part of a molecule. Within living systems, the atoms of Carbon-14 decay but they get replaced from the atmosphere so the ratio of Carbon-14 to Carbon-12 remains constant.

When the living system dies the Carbon-14 atoms stop being replaced. The ratio of Carbon-14 to Carbon-12 then begins to fall. By measuring this ratio, we can calculate the age of the object since it died. This is called Carbon Dating. A piece of wood can be dated to when it was cut down and used.

With radioactivity, if you have a number of radioactive atoms, half of them will decay in a period called the Half Life. Each type of radioactive atoms has its own unique half life. For Uranium the half life is billions of years (about the age of the Earth) making it useful for dating rocks. For Carbon-14, the half life is shorter, just 5,700 years. This makes Carbon Dating useful for dating human artefacts.

As you can see, Carbon is an interesting atom. 18% of a human body by mass is Carbon. 12% of all the atoms in your body are Carbon atoms. Carbon has been known since ancient times. In its diamond form it is one of the most sought after minerals. Its millions of compounds drive life on Earth and play a big role in the Earth's economy.

Carbon is truly a remarkable element.

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