Properties of covalent compounds and the versatility of carbon

Two kinds of compound, two kinds of behaviour

Put a spoon of salt and a spoon of wax in a hot pan. The wax melts quickly, and the salt does not change. Dip two wires in salt water, and a bulb lights. Dip them in kerosene, and it does not. The two kinds of compound behave in different ways, because they are held together in different ways.

Properties of covalent compounds

Compare sodium chloride with ethanol or wax.

• Low melting and boiling points. The atoms in a molecule are held by strong bonds, but the molecules attract each other only weakly. When a covalent compound melts or boils, the bonds inside the molecules do not break. Only the weak attraction between the molecules is overcome. Little energy is needed. So many covalent compounds are liquids or gases at room temperature, and the solids melt easily.
• Poor conductors of electricity. They have no ions, and no free electrons. There is nothing to carry the current.
• Soluble in organic solvents, such as petrol, and often not soluble in water. (Some, such as sugar and ethanol, do dissolve in water.)
• They burn, in many cases. Compounds of carbon and hydrogen burn in air.

There is one exception that you should remember. Diamond and graphite, in the last lesson, are covalent, and still have very high melting points. In them, the atoms are joined in a huge network of strong bonds, and there are no separate molecules.

You should find that the boiling point rises as the chain grows. Yet even octane boils far below the melting point of salt, $801\ ^\circ\text{C}$.

Why is carbon so versatile?

There are more than ten million known carbon compounds. All the other elements together make fewer than this. Carbon has two special properties.

1. Catenation. Carbon atoms can join to each other to make long chains, branched chains and rings. The C–C bond is very strong. Silicon, which is next below carbon, has much weaker Si–Si bonds, and it makes only short chains. Oxygen and nitrogen form chains of two or three atoms at most.

2. Tetravalency. Carbon has a valency of four. Each atom can bond to four other atoms. These can be other carbon atoms or atoms of hydrogen, oxygen, nitrogen, sulphur and chlorine.

Together, they allow an enormous number of structures. A chain of carbon atoms can be straight, branched, or closed into a ring, and each carbon atom can carry other atoms.

Compounds are also stable

The bonds of carbon are strong, so its compounds are stable. This is why we have long-lasting plastics, and why the molecules of our bodies do not fall apart. Carbon is the element of life. The proteins, the DNA, the sugars and the fats of every living thing are compounds of carbon.

More structures from the same atoms

When the number of carbon atoms in an alkane grows, the number of ways to join them grows very fast. Different structures with the same formula are called isomers. For butane, $\text{C}_4\text{H}_{10}$, there are two: a straight chain, and a chain with a branch.

Practise with new numbers

Try it in Python

This program lists the number of different structures that the alkanes can have.

Common mistakes

• Saying that the covalent bond breaks when a compound boils. Only the weak attraction between molecules is overcome.
• Saying that all covalent substances melt easily. Diamond has a very high melting point.
• Mixing up the two properties. Catenation is joining to the same element. Tetravalency is the number of bonds, four.
• Thinking that carbon is the most common element. It is not. It makes the most compounds.

Remember these

• Covalent compounds have low melting and boiling points, and do not conduct electricity.
• Catenation: carbon atoms join in chains, branches and rings, with strong C–C bonds.
• Tetravalency: carbon forms four bonds. These two properties give millions of compounds.
• Isomers have the same formula and different structures.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Kerosene does not light a bulb in a circuit. Why not?

2. When water boils, do the bonds between oxygen and hydrogen break?

3. Which two properties of carbon let it form millions of compounds?

4. Which is stronger: the bond between two carbon atoms, or the attraction between two separate molecules of methane?