Covalent bonds

Why carbon shares

In the last chapter, a metal gave electrons to a non-metal, and ions were made. Carbon cannot do this easily. It has four electrons in its outer shell, with the arrangement $2, 4$.

• To lose $4$ electrons and become $\text{C}^{4+}$, it would need a very large amount of energy.
• To gain $4$ electrons and become $\text{C}^{4-}$, the six protons would have to hold ten electrons, which is hard too.

So carbon shares electrons. Atoms of other non-metals, such as hydrogen, oxygen, nitrogen and chlorine, share electrons too.

The covalent bond

A covalent bond is formed when two atoms share a pair of electrons. Each atom counts the shared electrons as its own, and so it gets a full outer shell.

Hydrogen, $\text{H}_2$. Each hydrogen atom has one electron and needs two to fill its shell. Two atoms share one pair. Both now have two electrons. This is a single bond, written H–H.

Oxygen, $\text{O}_2$. Each oxygen has six outer electrons and needs two more. Two atoms share two pairs. This is a double bond, written O=O.

Nitrogen, $\text{N}_2$. Each nitrogen has five outer electrons and needs three more. Two atoms share three pairs. This is a triple bond, written N≡N.

Methane, $\text{CH}_4$. Carbon needs four. Each hydrogen needs one. Carbon shares one pair with each of the four hydrogen atoms: four single bonds.

Water, $\text{H}_2\text{O}$. Oxygen needs two and shares one pair with each of two hydrogen atoms.

Carbon dioxide, $\text{CO}_2$. Carbon needs four, and each oxygen needs two. Carbon shares two pairs with each oxygen: two double bonds, O=C=O.

Counting bonds from the valency

The valency of an atom is the number of bonds that it makes: hydrogen $1$, oxygen $2$, nitrogen $3$, carbon $4$. Each bond uses one unit of valency from each of its two atoms. So we can find the number of bonds:

For $\text{CH}_4$: $(4 + 4 \times 1) \div 2 = 4$ bonds. For $\text{H}_2\text{O}$: $(2 + 2 \times 1) \div 2 = 2$ bonds.

You should find that each carbon atom you add brings three more bonds: one C–C bond and two C–H bonds.

Practise with new numbers

Try it in Python

This program counts the bonds in some molecules.

Remember these

• A covalent bond is a shared pair of electrons. Each atom gets a full outer shell.
• Single bond: one pair. Double bond: two pairs. Triple bond: three pairs.
• Valency: H $1$, O $2$, N $3$, C $4$. Bonds $=$ (sum of valencies) $\div$ $2$.

Check yourself

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

1. Each chlorine atom has seven outer electrons. What kind of bond joins the two atoms of $\text{Cl}_2$?

2. How many bonds hold a molecule of chloromethane, $\text{CH}_3\text{Cl}$, together?

3. Why does a carbon atom not form the ion $\text{C}^{4+}$?

4. In methane, how many electrons does the carbon atom count in its outer shell?