Covalent Bonding and the Forms of Carbon: Quick Revision

A brief note on Covalent Bonding and the Forms of Carbon. Read it before a test. Try each question at the end before you open its answer.

Covalent bonds

A covalent bond is a pair of electrons shared by two atoms. Each atom counts the shared pair as its own, so both reach a full outer shell.

Valency: H $1$, O $2$, N $3$, C $4$. Shared pairs $=$ (sum of the valencies) $\div 2$.

Covalent compounds

• Made of molecules, held together by weak forces, so they melt and boil low.
• No ions and no free electrons, so they do not conduct.

Why carbon is special

• Catenation: carbon atoms join to one another in chains, branches and rings.
• Tetravalency: each carbon atom forms four bonds.

The forms of carbon

A worked example

How many pairs of electrons are shared in one molecule of propene, $\text{C}_3\text{H}_6$?

1. Valencies. $3 \times 4 + 6 \times 1 = 18$.
2. Shared pairs. $18 \div 2 = 9$.
3. Check. One C–C bond, one C=C bond (two pairs) and six C–H bonds: $1 + 2 + 6 = 9$.

Practice with new numbers

A program to try

The program works out the worked example, and two more.

Mistakes to avoid

• Saying that bonds break when a covalent compound boils. Only the weak forces between molecules are overcome.
• Counting a double bond as one pair. It is two shared pairs.
• Calling diamond and graphite different elements. They are allotropes of carbon.
• Thinking that all covalent substances melt low. Diamond melts very high.

Check yourself

1. How many electrons are shared in a triple bond?

2. Name the allotrope of carbon that is a hollow cage of atoms.

3. What is catenation?

4. How many bonds hold a molecule of ethane, $\text{C}_2\text{H}_6$, together?